Bug: POST Task Comment API doesn't works for some tasks

**Describe the bug**

The API to POST a task comment returns Internal Server Error (500) for some tasks and works as it should for some other.

Old Theory (most probably wrong):
This seems to be an issue with the hosting of the API on the server rather than the API itself. It has been observed that the API works perfectly when tested on a local server, and is only giving Internal Server Error on our server https://mentorship-backend-temp.herokuapp.com .

**To Reproduce**

Steps to reproduce the behavior:

1. Go to https://mentorship-backend-temp.herokuapp.com
2. Click on POST a new task comment under Mentorship Relations API
3. Click on Try it Out.
4. Enter valid token and ids.
5. Execute and see the error (on server). Also, the error isn't there when tested on local server.

**Expected behavior**
The API should work and post a new task comment. For a genuine error, the proper error message should be returned.

**Screenshots**

**Desktop (please complete the following information):**
- OS: [macOS]
- Browser [Safari]
- Version [13.1.1]