Fix crash with failed compatibility tiles

scene/resources/tile_set.cpp

@@ -2609,6 +2609,7 @@ void TileSet::_compatibility_conversion() {
 					compatibility_tilemap_mapping_tile_modes[E.key] = COMPATIBILITY_TILE_MODE_SINGLE_TILE;
 
 					TileData *tile_data = atlas_source->get_tile_data(coords, alternative_tile);
+					ERR_CONTINUE(!tile_data);
 
 					tile_data->set_flip_h(flip_h);
 					tile_data->set_flip_v(flip_v);