[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/askpassphrasedialog.h

@@ -27,7 +27,7 @@ public:
         Decrypt     /**< Ask passphrase and decrypt wallet */
     };
 
-    explicit AskPassphraseDialog(Mode mode, QWidget *parent = 0);
+    explicit AskPassphraseDialog(Mode mode, QWidget *parent);
     ~AskPassphraseDialog();
 
     void accept();