added 404 catching if no assignment exists

assignments/views.py

 from django.shortcuts import render
 from .models import Assignment, Course
+from django.http import Http404
 
 
 def ListView_view(request, *args, **kwargs):
@@ -10,7 +11,10 @@ def ListView_view(request, *args, **kwargs):
 
 
 def DetailView_view(request, pk):
+    if Assignment.objects.filter(id=pk).exists():
         detailed_context = {
             "assignment": Assignment.objects.get(id=pk)
         }
         return render(request, "assignments/DetailView.html", detailed_context)
+    else:
+        raise Http404('Assignment does not exist.')