Fix a crash after adding favourites to browser when the item is not y…

…et populated

The model was getting broken due to list of children being overwritten and
it was sometimes crashing in the proxy model implementation.

src/core/qgsdataitem.cpp

@@ -718,7 +718,8 @@ void QgsFavouritesItem::addDirectory( QString favDir )
   favDirs.append( favDir );
   settings.setValue( "/browser/favourites", favDirs );
 
-  addChildItem( new QgsDirectoryItem( this, favDir, favDir ), true );
+  if ( mPopulated )
+    addChildItem( new QgsDirectoryItem( this, favDir, favDir ), true );
 }
 
 void QgsFavouritesItem::removeDirectory( QgsDirectoryItem *item )
@@ -738,7 +739,8 @@ void QgsFavouritesItem::removeDirectory( QgsDirectoryItem *item )
     return;
   }
 
-  deleteChildItem( mChildren[idx] );
+  if ( mPopulated )
+    deleteChildItem( mChildren[idx] );
 }
 
 //-----------------------------------------------------------------------