Better errors regarding changing avatar_url

changelog.d/6497.bugfix

@@ -0,0 +1 @@
+Fix error message when setting your profile's avatar URL mentioning displaynames, and prevent NoneType avatar_urls.

synapse/rest/client/v1/profile.py

@@ -103,11 +103,16 @@ async def on_PUT(self, request, user_id):
 
         content = parse_json_object_from_request(request)
         try:
-            new_name = content["avatar_url"]
+            new_avatar_url = content.get("avatar_url")
         except Exception:
-            return 400, "Unable to parse name"
+            return 400, "Unable to parse avatar_url"
+
+        if new_avatar_url is None:
+            return 400, "Missing required key: avatar_url"
 
-        await self.profile_handler.set_avatar_url(user, requester, new_name, is_admin)
+        await self.profile_handler.set_avatar_url(
+            user, requester, new_avatar_url, is_admin
+        )
 
         return 200, {}