Fix Linux data-view dropdown popup parenting (#13721)

Delay opening the object-list filament dropdown on wxGTK until the editor
  window is mapped, avoiding popup creation without a valid native toplevel.
  Also set the GTK transient parent for dropdown popups created from data-view
  cell editors.

src/slic3r/GUI/ExtraRenderers.cpp

@@ -324,9 +324,18 @@ wxWindow* BitmapChoiceRenderer::CreateEditorCtrl(wxWindow* parent, wxRect labelR
     else
         c_editor->SetSelection(atoi(data.GetText().c_str()) - 1);
 
-    // Open the dropdown immediately when the editor is focused.
     c_editor->Bind(wxEVT_SET_FOCUS, [c_editor](wxFocusEvent& evt) {
+#ifdef __WXGTK__
+        // On wxGTK the data-view editor may receive focus before its native
+        // window is mapped. Opening the popup one event later avoids creating
+        // the GTK popup without a valid toplevel parent.
+        c_editor->CallAfter([c_editor]() {
+            if (c_editor->IsShownOnScreen())
+                c_editor->ForceDropdownOpen();
+        });
+#else
         c_editor->ForceDropdownOpen();
+#endif
         evt.Skip(); 
     });
 
@@ -392,4 +401,3 @@ wxSize TextRenderer::GetSize() const
     return GetTextExtent(m_value);
 }
 
-