Fix rendering HTML form when API error raised. Closes #2103.

Author: tomchristie

rest_framework/renderers.py

@@ -525,7 +525,10 @@ def get_rendered_html_form(self, data, view, method, request):
         else:
             instance = None
 
-        if request.method == method:
+        # If this is valid serializer data, and the form is for the same
+        # HTTP method as was used in the request then use the existing
+        # serializer instance, rather than dynamically creating a new one.
+        if request.method == method and serializer is not None:
             try:
                 data = request.data
             except ParseError: