[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/addressbookpage.h

@@ -39,7 +39,7 @@ public:
         ForEditing  /**< Open address book for editing */
     };
 
-    explicit AddressBookPage(Mode mode, Tabs tab, QWidget *parent = 0);
+    explicit AddressBookPage(Mode mode, Tabs tab, QWidget *parent);
     ~AddressBookPage();
 
     void setModel(AddressTableModel *model);