Ttyjfyk

In short:

I have the same problem, as is described here. However the suggested solutions do not work for me.

I managed to kill plasmashell via killall plasmashell and could then manually restart it using the command plasmashell.

Doing so resulted in the following messages to pop up on my screen:

ddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
No metadata file in the package, expected it at: "/usr/share/backgrounds/linuxmint-sylvia/"
No metadata file in the package, expected it at: "/usr/share/backgrounds/linuxmint-sylvia/"
No metadata file in the package, expected it at: "/usr/share/backgrounds/linuxmint-sylvia/"
trying to show an empty dialog
file:///usr/share/plasma/shells/org.kde.plasma.desktop/contents/views/Desktop.qml:147:19: QML Loader: Binding loop detected for property "height"
file:///usr/share/plasma/shells/org.kde.plasma.desktop/contents/views/Desktop.qml:147:19: QML Loader: Binding loop detected for property "height"
file:///usr/share/plasma/shells/org.kde.plasma.desktop/contents/views/Panel.qml:83: TypeError: Cannot read property 'Layout' of null
Connecting to deprecated signal QDBusConnectionInterface::serviceOwnerChanged(QString,QString,QString)
file:///usr/share/plasma/plasmoids/org.kde.plasma.taskmanager/contents/ui/main.qml:112:20: Unable to assign [undefined] to int
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
Trying to use rootObject before initialization is completed, whilst using setInitializationDelayed. Forcing completion
file:///usr/share/plasma/plasmoids/org.kde.plasma.digitalclock/contents/ui/main.qml:78:27: Unable to assign [undefined] to QStringList
file:///usr/share/plasma/plasmoids/org.kde.plasma.digitalclock/contents/ui/main.qml:37: TypeError: Cannot read property 'DateTime' of undefined
trying to show an empty dialog
trying to show an empty dialog
trying to show an empty dialog
file:///usr/share/plasma/plasmoids/org.kde.plasma.digitalclock/contents/ui/DigitalClock.qml:443:5: QML Text: Cannot anchor to a null item.
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")
kf5.kcoreaddons.desktopparser: Could not locate service type file kservicetypes5/plasma-popupapplet.desktop, tried ("/home/<userName>/.local/share", "/home/<userName>/.local/share/flatpak/exports/share", "/var/lib/flatpak/exports/share", "/usr/share/usr/share/xsessions/plasma", "/usr/local/share", "/usr/share")

I believe that most of this isn't too important but I guess the following lines have something to do with my problem:
trying to show an empty dialog

file:///usr/share/plasma/shells/org.kde.plasma.desktop/contents/views/Desktop.qml:147:19: QML Loader: Binding loop detected for property "height"
file:///usr/share/plasma/shells/org.kde.plasma.desktop/contents/views/Desktop.qml:147:19: QML Loader: Binding loop detected for property "height"
file:///usr/share/plasma/shells/org.kde.plasma.desktop/contents/views/Panel.qml:83: TypeError: Cannot read property 'Layout' of null

To me this indicates that there is some invalid data being passed around/created at some point that results in a null-value here which (I believe) is the root cause of the desktop not functioning well.

The longer story:

So I have Linux Mint 19 installed on my laptop on top of which I installed KDE. Everything was working just fine.

Today I went ahead and installed KDE neon on another partition on my laptop, leaving (in theory) the Mint-partition completely untouched.

The thing that got changed was my EFI-partition and the swap partition which both systems share.

It also seems as if the KDE Neon install has completely overridden the GRUB loader I had from Mint (Via BIOS I can select the loader of each system but the both result in the very same bootloader so I believe Neon simply overwrote the one of Mint).

I can boot into Mint just fine though. The only problem being that when I am logged in the splash screen takes way longer than before and after that I enter my desktop that now misses the start menu and the complete task bar. They're just gone. Right-clicking on my desktop doesn't do anything either.

Although I can see all my shortcuts on the desktop, I can't click any of them (more precisely: I can click on them but nothing happens).

EDIT:

I have thought about this problem a bit more: When the installer formatted the boot-partition, it also deleted the installed kernels. So maybe the problem is that the KDE-Neon kernel isn't uite compatible with the Linux Mint system which causes some trouble. I'll investigate further into that...

Your Problem was possibly caused by KDE-Neon overwriting desktop configuration files in your home directory ('/home/Your_User/.config/plasma-org.kde.plasma.desktop-appletsrc').

When you click on the shortcuts, they refer to applet definitions like this one:

[Containments][23][Applets][48][Configuration]
PreloadWeight=0
localPath=/home/me/.local/share/plasma_icons/chromium-browser.desktop
url=file:///usr/share/applications/chromium-browser.desktop

Your 'url=file:///' have been changed to refer to the mount point(s) of the neon partition, which doesn't exist when you load up mint.

If you have a backup of the file from BEFORE the Neon installation, overwrite the Neon-installed changes with the old file. If you don't... you can hand-edit the file. Or, you might have to delete everything and build it new (new panels, new desktop shortcuts). Sorry! But be careful running different Distros with the same software, sharing the same home partition and directories. ALWAYS BACKUP FIRST.

When trying a new 'live' Distro, you can also create a new UserID within /home - logging in as the new user (with empty config files). With this method, the Live Distro will write it's default config files into the new UserID, rather than overwriting your production 'User' information.

Here is a snip of text, from Mageia's "live DVD" warning section, describing the issue with their own "Live" media versions:

Warning!
Create a user with a new login if you don't format a /home
partition. User's configuration is written in user's space,
which can overwrite some personal data. This can be the case for
Firefox, Thunderbird, Kmail data...

... and this will apply to your .config settings, as well.

You installed Neon in a separate partition (containing system root root "/", "/var", "/usr", and swap). But, as with Mageiea-Live, if it recognizes an existing /home partition and you use it, with an already-configured existing login ... it will edit and overwrite files within your Mint UserID configuration.

Slightly OT: It seems 'unfriendly' that KDE-NEON can't "install" without destroying the GRUB2 information for other Distros... but that is a different problem from the .config over-writing.