fix

rowers/models.py

@@ -1242,7 +1242,7 @@ class DeleteUserForm(forms.ModelForm):
         fields = []
 
 class UserMessage(models.Model):
-    receiver = models.ForeignKey(Rower, on_delete=models.CASCADE)
+    receiver = models.ForeignKey(Rower, blank=True, null=True, on_delete=models.SET_NULL)
     datetime = models.DateTimeField()
     isread = models.BooleanField(default=False)
     text = models.CharField(max_length=1000)