Description of problem:
Editing MATE panel menu items is not possible and causes a core dump.

Version-Release number of selected component (if applicable):
mate-panel-1.26.0-1.fc34.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Right-click MATE panel applications menu
2. Click Edit Menus
3. Highlight menu item
4. Click Properties button or double click menu item

Actual results:
No Properties dialog appears (eg. to edit the command for that menu item) and the system log records a core dump.


Expected results:
Have an editable Properties dialog appear.

Additional info:

systemd-coredump[77431]: [🡕] Process 77426 (mate-desktop-it) of user 1000 dumped core.
                                              
                                              Stack trace of thread 77426:
                                              #0  0x00007f67300c42a2 raise (libc.so.6 + 0x3d2a2)
                                              #1  0x00007f67300ad8a4 abort (libc.so.6 + 0x268a4)
                                              #2  0x00007f67303ddccc g_assertion_message.cold (libglib-2.0.so.0 + 0x1eccc)
                                              #3  0x00007f673043930f g_assertion_message_expr (libglib-2.0.so.0 + 0x7a30f)
                                              #4  0x000055b7df9716a9 panel_ditem_editor_sync_display (mate-desktop-item-edit + 0x136a9)
                                              #5  0x000055b7df971cfa panel_ditem_editor_key_file_loaded (mate-desktop-item-edit + 0x13cfa)
                                              #6  0x000055b7df9728cc panel_ditem_editor_constructor (mate-desktop-item-edit + 0x148cc)
                                              #7  0x00007f673051bae8 g_object_new_internal (libgobject-2.0.so.0 + 0x22ae8)
                                              #8  0x00007f673051d108 g_object_new_valist (libgobject-2.0.so.0 + 0x24108)
                                              #9  0x00007f673051d63d g_object_new (libgobject-2.0.so.0 + 0x2463d)
                                              #10 0x000055b7df96bade panel_ditem_editor_new_full (mate-desktop-item-edit + 0xdade)
                                              #11 0x000055b7df96a082 main (mate-desktop-item-edit + 0xc082)
                                              #12 0x00007f67300aeb75 __libc_start_main (libc.so.6 + 0x27b75)
                                              #13 0x000055b7df96a1ce _start (mate-desktop-item-edit + 0xc1ce)
                                              
                                              Stack trace of thread 77427:
                                              #0  0x00007f673017c5bf __poll (libc.so.6 + 0xf55bf)
                                              #1  0x00007f673046848c g_main_context_iterate.constprop.0 (libglib-2.0.so.0 + 0xa948c)
                                              #2  0x00007f6730411c03 g_main_context_iteration (libglib-2.0.so.0 + 0x52c03)
                                              #3  0x00007f6730411c51 glib_worker_main (libglib-2.0.so.0 + 0x52c51)
                                              #4  0x00007f6730442c42 g_thread_proxy (libglib-2.0.so.0 + 0x83c42)
                                              #5  0x00007f673025f299 start_thread (libpthread.so.0 + 0x9299)
                                              #6  0x00007f6730187353 __clone (libc.so.6 + 0x100353)
                                              
                                              Stack trace of thread 77428:
                                              #0  0x00007f673017c5bf __poll (libc.so.6 + 0xf55bf)
                                              #1  0x00007f673046848c g_main_context_iterate.constprop.0 (libglib-2.0.so.0 + 0xa948c)
                                              #2  0x00007f6730413a93 g_main_loop_run (libglib-2.0.so.0 + 0x54a93)
                                              #3  0x00007f6730666d9a gdbus_shared_thread_func.lto_priv.0 (libgio-2.0.so.0 + 0x110d9a)
                                              #4  0x00007f6730442c42 g_thread_proxy (libglib-2.0.so.0 + 0x83c42)
                                              #5  0x00007f673025f299 start_thread (libpthread.so.0 + 0x9299)
                                              #6  0x00007f6730187353 __clone (libc.so.6 + 0x100353)


abrt-notification[77519]: [🡕] Process 2648104 (mate-desktop-item-edit) crashed in g_assertion_message_expr.cold()