fix issue for weird error in first question due to the systematic run…

…ning of postcmd

madgraph/interface/extended_cmd.py

@@ -1113,7 +1113,7 @@ def ask(self, question, default, choices=[], path_msg=None,
         question_instance = obj(question, allow_arg=choices, default=default, 
                                                    mother_interface=self, **opt)
         if fct_timeout is None:
-            fct_timeout = lambda x: question_instance.postcmd(x, default) if x else False
+            fct_timeout = lambda x: question_instance.postcmd(x, default) if x and default else False
         if first_cmd:
             if isinstance(first_cmd, str):
                 question_instance.onecmd(first_cmd)
@@ -2274,6 +2274,9 @@ def postcmd(self, stop, line):
                 if n:
                     self.default(line)
                     return self.postcmd(stop, line)
+            elif self.value is None and line:
+                self.default(line)
+                return self.postcmd(stop, line) 
             if not self.casesensitive:
                 for ans in self.allow_arg:
                     if ans.lower() == self.value.lower():