Only write wallet if necessary

gui/qt/main_window.py

@@ -282,8 +282,9 @@ class ElectrumWindow(QMainWindow, PrintError):
 
     def import_old_contacts(self):
         # backward compatibility: import contacts
-        addressbook = set(self.wallet.storage.get('contacts', []))
+        old_contacts = self.wallet.storage.get('contacts', [])
-        for k in addressbook:
+        if old_contacts:
+            for k in set(old_contacts):
                 l = self.wallet.labels.get(k)
                 if bitcoin.is_address(k) and l:
                     self.contacts[l] = ('address', k)