check for null event in Viewport::push_input

scene/main/viewport.cpp

@@ -2904,6 +2904,7 @@ bool Viewport::_sub_windows_forward_input(const Ref<InputEvent> &p_event) {
 
 void Viewport::push_input(const Ref<InputEvent> &p_event, bool p_local_coords) {
 	ERR_FAIL_COND(!is_inside_tree());
+	ERR_FAIL_COND(p_event.is_null());
 
 	if (disable_input) {
 		return;