Fix SceneTreeEditor crashing when calling _deselect_items

The crash can be triggered for instance by trying to reparent a node
and clicking on the background of the Reparent Node window.

editor/scene_tree_editor.cpp

@@ -484,6 +484,7 @@ void SceneTreeEditor::_selected_changed() {
 void SceneTreeEditor::_deselect_items() {
 
 	// Clear currently elected items in scene tree dock.
+	if (editor_selection)
 		editor_selection->clear();
 }