fix total being not zero when adding a new resolution

fet2020/finance/forms.py

@@ -439,21 +439,19 @@ class ResolutionAdminForm(forms.ModelForm):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)  # to get the self.fields set
 
-        resolution = kwargs.get("instance")
-        bills = Bill.objects.filter(resolution=resolution)
-        total = 0
-        for elem in bills:
+        budget = 0.0
+        total = 0.0
+        if (resolution := kwargs.get("instance")) is not None:
+            for elem in Bill.objects.filter(resolution=resolution):
                 total += elem.amount
 
+            budget = resolution.budget
+
         self.fields["total"].disabled = True
         self.fields["total"].initial = total
         self.fields["total"].label = "Gesamtsumme (EUR)"
         self.fields["total"].required = False
 
-        budget = 0
-        if resolution:
-            budget = resolution.budget
-
         self.fields["budget_remaining"].disabled = True
         self.fields["budget_remaining"].initial = budget - total
         self.fields["budget_remaining"].label = "Restbudget (EUR)"