[Qt] make most Windows appear centered on main GUI

- remove default value for parent variable in dialogs, which should appear
  centered over main GUI and pass appropriate value
- add some addition NULL pointer checks

src/qt/optionsdialog.h

@@ -21,7 +21,7 @@ class OptionsDialog : public QDialog
     Q_OBJECT
 
 public:
-    explicit OptionsDialog(QWidget *parent = 0);
+    explicit OptionsDialog(QWidget *parent);
     ~OptionsDialog();
 
     void setModel(OptionsModel *model);