I was trying to simulate the "Error saving base view: null" error in my Virtual DataPort instance by performing the Source Refresh option over a base view. In my case, the source refresh was working as expected.
For your scenario, I would try the following to identify the root cause:
* Check the vdp.log file located under the directory ‘**<DENODO_HOME>/logs/vdp**’ to find more detailed errors if any.
* Try performing a source refresh on base view created over another data source and see if this issue is specific to Custom Data Source.
* If the above succeeds, I would review the Custom Data Source code to check for any issues.
If you are a valid support user then you can raise a Support case in the [Denodo Support Site](https://support.denodo.com/) so that our Support Team will assist you.
Hope this helps!