fix multiclassing

ttfrog/webserver/forms.py

@@ -1,6 +1,7 @@
 from ttfrog.db.manager import db
 from wtforms.fields import SelectField, SelectMultipleField
 
+
 class DeferredSelectMultipleField(SelectMultipleField):
     def __init__(self, *args, model=None, **kwargs):
         super().__init__(*args, **kwargs)
@@ -11,3 +12,9 @@ class DeferredSelectField(SelectField):
     def __init__(self, *args, model=None, **kwargs):
         super().__init__(*args, **kwargs)
         self.choices = [(rec.id, rec.name) for rec in db.query(model).all()]
+
+
+class NullableDeferredSelectField(DeferredSelectField):
+    def __init__(self, *args, model=None, label='---',  **kwargs):
+        super().__init__(*args, model=model,  **kwargs)
+        self.choices = [(0, label)] + self.choices