define a setMap for current_metatile_selection_item to prevent crash on project transition

include/ui/currentselectedmetatilespixmapitem.h

@@ -15,6 +15,8 @@ public:
     Map* map = nullptr;
     MetatileSelector *metatileSelector;
     void draw();
+
+    void setMap(Map *map) { this->map = map; }
 };
 
 #endif // CURRENTSELECTEDMETATILESPIXMAPITEM_H

src/editor.cpp

@@ -1299,6 +1299,7 @@ void Editor::displayCurrentMetatilesSelection() {
 
 void Editor::redrawCurrentMetatilesSelection() {
     if (current_metatile_selection_item) {
+        current_metatile_selection_item->setMap(map);
         current_metatile_selection_item->draw();
         emit currentMetatilesSelectionChanged();
     }