Rasa validator shows empty error message when text: null for a response

[desmarchris]

Rasa Version     : 2.2.2
Rasa SDK Version : 2.2.0
Rasa X Version   : None
Python Version   : 3.8.6
Operating System : macOS-10.16-x86_64-i386-64bit

Issue:
Originally reported in our forum, training/validating threw an error but didn't show anything in the file to fix. After looking at the file, one of the responses had text: null. Changing this to a random string fixed the problem. We should look into why this error wasn't showing up in the traceback

Error (including full traceback):

(e2e) chris@ChristophersMBP e2e % rasa data validate -d original-domain.yml
The configuration for pipeline was chosen automatically. It was written into the config file at 'config.yml'.
YamlValidationException: Failed to validate '/Users/chris/rasa/e2e/original-domain.yml'. Please make sure the file is correct and all mandatory parameters are specified. Here are the errors found during validation

Command or request that led to error:

rasa train

rasa validate

Content of domain file (domain.yml) (if relevant):
File is here

Definition of Done

• YamlValidationException should show what made the schema validation fail

[desmarchris]

there have been a few more cases in this forum thread. The latest one was due to still including the format for MappingPolicy and from a response formatted with an extra tick:

  utter_cheer_up:
    - image: https://i.imgur.com/nGF1K8f.jpg
    - text: Here is something to cheer you up

Here's the domain file

rasa data validate just outputs this:

(rasa) ➜  test rasa data validate --domain domain2.yml
YamlValidationException: Failed to validate '/Users/chris/Downloads/test/domain2.yml'. Please make sure the file is correct and all mandatory parameters are specified. Here are the errors found during validation

[ancalita]

PR merged to 2.6.x