[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.cpp

@@ -158,6 +158,9 @@ void AddressBookPage::onCopyLabelAction()
 
 void AddressBookPage::onEditAction()
 {
+    if(!model)
+        return;
+
     if(!ui->tableView->selectionModel())
         return;
     QModelIndexList indexes = ui->tableView->selectionModel()->selectedRows();
@@ -165,9 +168,9 @@ void AddressBookPage::onEditAction()
         return;
 
     EditAddressDialog dlg(
-            tab == SendingTab ?
-            EditAddressDialog::EditSendingAddress :
-            EditAddressDialog::EditReceivingAddress);
+        tab == SendingTab ?
+        EditAddressDialog::EditSendingAddress :
+        EditAddressDialog::EditReceivingAddress, this);
     dlg.setModel(model);
     QModelIndex origIndex = proxyModel->mapToSource(indexes.at(0));
     dlg.loadRow(origIndex.row());
@@ -180,9 +183,9 @@ void AddressBookPage::on_newAddress_clicked()
         return;
 
     EditAddressDialog dlg(
-            tab == SendingTab ?
-            EditAddressDialog::NewSendingAddress :
-            EditAddressDialog::NewReceivingAddress, this);
+        tab == SendingTab ?
+        EditAddressDialog::NewSendingAddress :
+        EditAddressDialog::NewReceivingAddress, this);
     dlg.setModel(model);
     if(dlg.exec())
     {