Check for null event in Tree._gui_input to avoid engine crash

scene/gui/tree.cpp

@@ -2175,6 +2175,8 @@ void Tree::_go_down() {
 }
 
 void Tree::_gui_input(Ref<InputEvent> p_event) {
+	ERR_FAIL_COND(p_event.is_null());
+
 	Ref<InputEventKey> k = p_event;
 
 	bool is_command = k.is_valid() && k->get_command();