[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/editaddressdialog.h

@@ -31,7 +31,7 @@ public:
         EditSendingAddress
     };
 
-    explicit EditAddressDialog(Mode mode, QWidget *parent = 0);
+    explicit EditAddressDialog(Mode mode, QWidget *parent);
     ~EditAddressDialog();
 
     void setModel(AddressTableModel *model);