Fix crash on startup when python support is not present

Adds a safety check for mActionShowPythonDialog before accessing it

Author: m-kuhn

src/app/qgisapp.cpp

@@ -738,7 +738,7 @@ QgisApp::QgisApp( QSplashScreen *splash, bool restorePlugins, QWidget * parent,
     // enable Python in the Plugin Manager and pass the PythonUtils to it
     mPluginManager->setPythonUtils( mPythonUtils );
   }
-  else
+  else if ( mActionShowPythonDialog )
   {
     mActionShowPythonDialog->setVisible( false );
   }