Qbs: Fix build after recent QmlJs refactoring

Change-Id: I049deda46c8cb8bbe4bf15000315c4271c05e11c
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/qbsprojectmanager/qbsproject.cpp

@@ -605,6 +605,10 @@ void QbsProject::updateCppCodeModel(const qbs::ProjectData *prj)
 void QbsProject::updateQmlJsCodeModel(const qbs::ProjectData *prj)
 {
     Q_UNUSED(prj);
+    QmlJS::ModelManagerInterface *modelManager = QmlJS::ModelManagerInterface::instance();
+    if (!modelManager)
+        return;
+
     QmlJS::ModelManagerInterface::ProjectInfo projectInfo =
             QmlJSTools::defaultProjectInfoForProject(this);
     modelManager->updateProjectInfo(projectInfo);