[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

1 files changed, 1 insertions, 1 deletions

diff --git a/src/qt/aboutdialog.h b/src/qt/aboutdialog.h
index b02be74844..1b131c4dcc 100644
--- a/src/qt/aboutdialog.h
+++ b/src/qt/aboutdialog.h
@@ -19,7 +19,7 @@ class AboutDialog : public QDialog
     Q_OBJECT
 
 public:
-    explicit AboutDialog(QWidget *parent = 0);
+    explicit AboutDialog(QWidget *parent);
     ~AboutDialog();
 
     void setModel(ClientModel *model);