StudioWelcome: Fix crash

We have to if the project actually has a QmlBuildSystem.

Task-number: QDS-9804
Change-Id: I1740f1c0772641888a8828b10380f442308bfe46
(cherry picked from commit 5f3c91180f7f5ecd2bfa15f11b3d5742f1e26ea4)
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/studiowelcome/studiowelcomeplugin.cpp

@@ -113,6 +113,9 @@ static Utils::FilePath getMainUiFileWithFallback()
     auto qmlBuildSystem = qobject_cast<QmlProjectManager::QmlBuildSystem *>(
         project->activeTarget()->buildSystem());
 
+    if (!qmlBuildSystem)
+        return {};
+
     auto mainUiFile = qmlBuildSystem->mainUiFilePath();
     if (mainUiFile.exists())
         return mainUiFile;