Added useful error message

[AlexejPenner]

Describe changes

The following error message might be a bit confusing to newcomers:

AttributeError: 'NoneType' object has no attribute 'name'

Instead I am now manually raising an error explicitly that tells the user whats up.

Pre-requisites

Please ensure you have done the following:

• If my change requires a change to docs, I have updated the documentation accordingly.
• I have added tests to cover my changes.
• I have updated ref-zenml in .github/workflows/ci.yml accordingly (if you don't know - main would be a solid choice)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (add details above)