Avoid crash if no project file is selected and OK is clicked

(cherry picked from commit 6318f67)

Author: wonder-sk

src/app/qgisapp.cpp

@@ -8032,7 +8032,7 @@ void QgisApp::embedLayers()
 {
   //dialog to select groups/layers from other project files
   QgsProjectLayerGroupDialog d( this );
-  if ( d.exec() == QDialog::Accepted )
+  if ( d.exec() == QDialog::Accepted && d.isValid() )
   {
     mMapCanvas->freeze( true );
 

src/app/qgsprojectlayergroupdialog.cpp

@@ -102,6 +102,11 @@ QString QgsProjectLayerGroupDialog::selectedProjectFile() const
   return mProjectFileLineEdit->text();
 }
 
+bool QgsProjectLayerGroupDialog::isValid() const
+{
+  return mTreeView->layerTreeModel() != 0;
+}
+
 void QgsProjectLayerGroupDialog::on_mBrowseFileToolButton_clicked()
 {
   //line edit might emit editingFinished signal when loosing focus

src/app/qgsprojectlayergroupdialog.h

@@ -36,6 +36,8 @@ class APP_EXPORT QgsProjectLayerGroupDialog: public QDialog, private Ui::QgsProj
     QStringList selectedLayerNames() const;
     QString selectedProjectFile() const;
 
+    bool isValid() const;
+
   private slots:
     void on_mBrowseFileToolButton_clicked();
     void on_mProjectFileLineEdit_editingFinished();