Fix crash when hiding subwindow during popup of new subwindow

(cherry picked from commit e2cea458a34665c03d57aa52bddcfd8054d3bf03)

scene/main/viewport.cpp

@@ -364,6 +364,7 @@ void Viewport::_sub_window_grab_focus(Window *p_window) {
 		return;
 	}
 
+	// The index needs to be update before every usage in case an event callback changed the window list.
 	int index = _sub_window_find(p_window);
 	ERR_FAIL_COND(index == -1);
 
@@ -375,6 +376,8 @@ void Viewport::_sub_window_grab_focus(Window *p_window) {
 			gui.subwindow_drag = SUB_WINDOW_DRAG_DISABLED;
 		}
 		// Can only move to foreground, but no focus granted.
+		index = _sub_window_find(p_window);
+		ERR_FAIL_COND(index == -1);
 		SubWindow sw = gui.sub_windows[index];
 		gui.sub_windows.remove_at(index);
 		gui.sub_windows.push_back(sw);
@@ -402,6 +405,8 @@ void Viewport::_sub_window_grab_focus(Window *p_window) {
 	gui.subwindow_focused->_event_callback(DisplayServer::WINDOW_EVENT_FOCUS_IN);
 
 	{ // Move to foreground.
+		index = _sub_window_find(p_window);
+		ERR_FAIL_COND(index == -1);
 		SubWindow sw = gui.sub_windows[index];
 		gui.sub_windows.remove_at(index);
 		gui.sub_windows.push_back(sw);