QmlDesigner: Fix crash

The buildsystem might be missing.

Change-Id: I05e03f59a2fcb49ce1ff7a2521ed1af165730b51
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/qmldesigner/components/toolbar/toolbarbackend.cpp

@@ -64,6 +64,10 @@ static Utils::FilePath getMainUiFile()
 
     auto qmlBuildSystem = qobject_cast<QmlProjectManager::QmlBuildSystem *>(
         project->activeTarget()->buildSystem());
+
+    if (!qmlBuildSystem)
+        return {};
+
     return qmlBuildSystem->mainUiFilePath();
 }