Fixed crash on applying or changing settings

The m_page member used to access the widgets was deleted too early.
Deletion moved to finish() method.

src/plugins/coreplugin/generalsettings.h

@@ -56,7 +56,7 @@ public:
     QString trCategory() const;
     QWidget* createPage(QWidget *parent);
     void apply();
-    void finish() { }
+    void finish();
 
 private slots:
     void resetInterfaceColor();