[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/openuridialog.h

@@ -16,7 +16,7 @@ class OpenURIDialog : public QDialog
     Q_OBJECT
 
 public:
-    explicit OpenURIDialog(QWidget *parent = 0);
+    explicit OpenURIDialog(QWidget *parent);
     ~OpenURIDialog();
 
     QString getURI();