Ttyjfyk

I use a dialog-bubble style for some theorems, by using the code below, that works fine.

documentclass[a4paper,11pt,twoside]book
usepackage[a4paper,left=2.5cm,right=2.5cm,top=2.5cm,bottom=2.5cm]geometry
usepackageamsthm

usepackageenviron

usepackagetikz
usetikzlibraryshapes,snakes % Για φάνσι κουτακια

theoremstyleplain
newtheoremtheoSometheorem[chapter]

tikzstyleBoxDialog = [draw=black, fill=white, very thick,
rectangle callout, rounded corners, densely dashed,callout relative pointer=(-0.4cm,-0.4cm), inner sep=4pt, inner ysep=8pt]
tikzstyleTitlTheo =[fill=white, text=black]

NewEnvirondialogtheo[1]
 begintikzpicture
 node [BoxDialog] (boxtheo)%
 beginminipage0.9textwidth
 BODY
 endminipage
 ;
 node[TitlTheo] at (boxtheo.north) textbf#1;
 endtikzpicture


begindocument

begindialogtheotitle theorem
begintheo
 theorem theorem theorem theorem theorem theorem theorem theorem theorem theorem
endtheo
enddialogtheo

enddocument

But when I wrap center environment around the dialogtheo environment like this:

NewEnvirondialogtheo[1]
 begincenter
 begintikzpicture
 node [BoxDialog] (boxtheo)%
 beginminipage0.9textwidth
 BODY
 endminipage
 ;
 node[TitlTheo] at (boxtheo.north) textbf#1;
 endtikzpicture
 endcenter

I have this error report:

! LaTeX Error: Something's wrong--perhaps a
missing item.

This is the log-file.

What is the problem?

P.S.: If I ceep center environment, but I delete theo, everything is OK again.

If your goal is to center the dialog bubble, instead of using the center environment, you should try to use centering with a par at the end of the content of your environment:

NewEnvirondialogtheo[1]
 centering
 begintikzpicture
 node[BoxDialog] (boxtheo)
 beginminipage0.9textwidth
 BODY
 endminipage
 ;
 node[TitlTheo] at (boxtheo.north) textbf#1;
 endtikzpicturepar

The par is important, else the bubble will not be centered at all.

If you try to place a theorem inside center environment, you will realize that it doesn't work.

Edit: I mean that the code:

begincenter
 begintheo
 test
 endtheo
endcenter

would result to a non-centered theorem.

So, my solution is a "hack" that just indenting your environment as much as needed:

documentclass[a4paper,11pt,twoside]book
usepackage[a4paper,left=2.5cm,right=2.5cm,top=2.5cm,bottom=2.5cm]geometry
usepackageamsthm
usepackagelipsum
usepackageenviron

usepackagetikz
usetikzlibraryshapes,decorations % Για φάνσι κουτακια

theoremstyleplain
newtheoremtheoSometheorem[chapter]

tikzstyleBoxDialog = [draw=black, fill=white, very thick,
rectangle callout, rounded corners, densely dashed,callout relative pointer=(-0.4cm,-0.4cm), inner sep=4pt, inner ysep=8pt]
tikzstyleTitlTheo =[fill=white, text=black]

newsaveboxtheobox
newlengththeoidentlength
NewEnvirondialogtheo[1]
saveboxtheoboxhbox
 begintikzpicture
 node [BoxDialog] (boxtheo)%
 beginminipage0.6textwidth
 BODY
 endminipage
 ;
 node[TitlTheo] at (boxtheo.north) textbf#1;
 endtikzpicture
 setlengththeoidentlengthdimexpr(textwidth-wdtheobox)/2relax
 noindenthspace*theoidentlengthuseboxtheobox


begindocument
lipsum[1]
begindialogtheotitle theorem

begintheo
 theorem theorem theorem theorem theorem theorem theorem theorem theorem theorem
endtheo
enddialogtheo

enddocument

PS: My solution is just an alternative [that I think could be useful in many cases if centering fails], and just added as such (an alternative) approach.