Qbs: Avoid crash when asking for qbs::ProjectData on invalid Project

Change-Id: I2ce060ab0017eb7722b6dc4f64b1e536f139952c
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/qbsprojectmanager/qbsnodes.cpp

@@ -834,7 +834,10 @@ const qbs::Project QbsProjectNode::qbsProject() const
 
 const qbs::ProjectData QbsProjectNode::qbsProjectData() const
 {
+    if (m_qbsProject.isValid())
         return m_qbsProject.projectData();
+    else
+        return qbs::ProjectData();
 }
 
 bool QbsProjectNode::showInSimpleTree() const