default to tab if user cancels delimeter selection

aqt/importing.py

@@ -99,7 +99,7 @@ class ImportDialog(QDialog):
 By default, Anki will detect the character between fields, such as
 a tab, comma, and so on. If Anki is detecting the character incorrectly,
 you can enter it here. Use \\t to represent tab."""),
-                self, help="importing")
+                self, help="importing") or "\t"
         str = str.replace("\\t", "\t")
         str = str.encode("ascii")
         self.hideMapping()