[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/signverifymessagedialog.h

@@ -18,7 +18,7 @@ class SignVerifyMessageDialog : public QDialog
     Q_OBJECT
 
 public:
-    explicit SignVerifyMessageDialog(QWidget *parent = 0);
+    explicit SignVerifyMessageDialog(QWidget *parent);
     ~SignVerifyMessageDialog();
 
     void setModel(WalletModel *model);