Fix crash in Meshviewer when dialog-window was closed before opening model (found by randomMesh).

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3035 dfc29bdd-3216-0410-991c-e03cc46cb475

examples/09.Meshviewer/main.cpp

@@ -128,6 +128,8 @@ void SetSkinTransparency(s32 alpha, irr::gui::IGUISkin * skin)
 void UpdateScaleInfo(scene::ISceneNode* model)
 {
 	IGUIElement* toolboxWnd = Device->getGUIEnvironment()->getRootGUIElement()->getElementFromId(GUI_ID_DIALOG_ROOT_WINDOW, true);
+	if (!toolboxWnd)
+		return;
 	if (!model)
 	{
 		toolboxWnd->getElementFromId(GUI_ID_X_SCALE, true)->setText( L"-" );