bug fix: empty field in allprojects form

app/vkk/workhours/allprojects/forms.py

@@ -91,7 +91,7 @@ class AssigneeForm(forms.Form):
                             project_assignment=assignment,
                             period=period,
                         )         
-                        value = self.cleaned_data[field_name] - (workhours.aggregate(Sum('hours'))['hours__sum'] or 0)
+                        value = (self.cleaned_data[field_name] or 0) - (workhours.aggregate(Sum('hours'))['hours__sum'] or 0)
                         if value == 0:
                             WorkHoursCorrection.objects.filter(
                                 project_assignment=assignment,