fix: need to be string in defaultFile

Signed-off-by: yihong0618 <zouzou0208@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: yihong0618

Lib/pdb.py

@@ -1413,9 +1413,9 @@ def do_break(self, arg, temporary=False):
     def defaultFile(self):
         """Produce a reasonable default."""
         if self.curframe is None:
-            self.error("No current frame.")
-            return None
-        filename = self.curframe.f_code.co_filename
+            filename = '<string>'
+        else:
+            filename = self.curframe.f_code.co_filename
         if filename == '<string>' and self.mainpyfile:
             filename = self.mainpyfile
         return filename