[dataflow][qtgui] fix crash when ending graph edition session

src/Dataflow/QtGui/GraphEditor/GraphEditorWindow.cpp

@@ -8,13 +8,18 @@ namespace GraphEditor {
 using namespace Ra::Dataflow::Core;
 
 GraphEditorWindow::~GraphEditorWindow() {
-    // Prevent graph destruction if the editor is used to work with active graphs.
-    auto graphProtection = m_graph->getNodesAndLinksProtection();
-    if ( !m_ownGraph ) { m_graph->setNodesAndLinksProtection( true ); }
-    delete m_graphEdit;
-    if ( !m_ownGraph ) { m_graph->setNodesAndLinksProtection( graphProtection ); }
+    if ( m_graph ) {
+        // Prevent graph destruction if the editor is used to work with active graphs.
+        auto graphProtection = m_graph->getNodesAndLinksProtection();
+        if ( !m_ownGraph ) { m_graph->setNodesAndLinksProtection( true ); }
+        delete m_graphEdit;
+        if ( !m_ownGraph ) { m_graph->setNodesAndLinksProtection( graphProtection ); }
+        else {
+            delete m_graph;
+        }
+    }
     else {
-        delete m_graph;
+        delete m_graphEdit;
     }
 }
 
@@ -101,6 +106,7 @@ bool GraphEditorWindow::saveAs() {
     QFileDialog dialog( this );
     dialog.setWindowModality( Qt::WindowModal );
     dialog.setAcceptMode( QFileDialog::AcceptSave );
+    dialog.setDefaultSuffix( "json" );
     if ( dialog.exec() != QDialog::Accepted ) return false;
     return saveFile( dialog.selectedFiles().first() );
 }