Fix error when export is cancelled without specifying a filename.

src/qt/addressbookpage.cpp

@@ -187,6 +187,8 @@ void AddressBookPage::exportClicked()
             QDir::currentPath(),
             tr("Comma separated file (*.csv)"));
 
+    if (filename.isNull()) return;
+
     CSVModelWriter writer(filename);
 
     // name, column, role

src/qt/transactionview.cpp

@@ -242,6 +242,8 @@ void TransactionView::exportClicked()
             QDir::currentPath(),
             tr("Comma separated file (*.csv)"));
 
+    if (filename.isNull()) return;
+
     CSVModelWriter writer(filename);
 
     // name, column, role