Fix #127539: Prevent multi-object editing in GPv3

Multi-object editing is not supported in GPv3 right now,
do not allow multiple GPv3 objects to go into edit mode
at the same time.

Pull Request: https://projects.blender.org/blender/blender/pulls/127796

source/blender/editors/object/object_edit.cc

@@ -936,7 +936,8 @@ static int editmode_toggle_exec(bContext *C, wmOperator *op)
 
   if (!is_mode_set) {
     editmode_enter_ex(bmain, scene, obact, 0);
-    if (obact->mode & mode_flag) {
+    /* Grease Pencil does not support multi-object editing. */
+    if ((obact->type != OB_GREASE_PENCIL) && ((obact->mode & mode_flag) != 0)) {
       FOREACH_SELECTED_OBJECT_BEGIN (view_layer, v3d, ob) {
         if ((ob != obact) && (ob->type == obact->type)) {
           editmode_enter_ex(bmain, scene, ob, EM_NO_CONTEXT);