Protect potential null pointer access with g_return_if_fail().
Remaining warnings show the unfinished state of focus handling:

textedit.c: In function ‘textedit_end_edit’:
textedit.c:148:19: warning: unused variable ‘change’ [-Wunused-variable]
   TextEditChange *change;
                   ^
textedit.c: In function ‘textedit_remove_focus’:
textedit.c:303:10: warning: unused variable ‘old_focus’ [-Wunused-variable]
   Focus *old_focus = get_active_focus((DiagramData *) diagram);
          ^
textedit.c: At top level:
textedit.c:370:1: warning: ‘text_edit_create_change’ defined but not used [-Wunused-function]
 text_edit_create_change(Text *text)
 ^
textedit.c:394:1: warning: ‘text_edit_update’ defined but not used [-Wunused-function]
 text_edit_update(TextEditChange *change)
 ^
textedit.c:303:10: warning: Value stored to 'old_focus' during its initialization is never read
  Focus *old_focus = get_active_focus((DiagramData *) diagram);
         ^~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.