Top Tip: Drupal Views - No valid values found on filter

Expose your Master display!

Now and again a little issue occurs that makes you pull your hair out.

I like to have NO errors on my sites and today whilst refactoring a view kept getting this error: No valid values found on filter: Content. But all my filters seem OK and the views work fine....

Spent ages trying to track it down - going through the filters removing them re creating etc. Exporting the view and scouring the export.
Then came across this top tip: from Robertom  - although to be honest I found it here  from gerhard1176 who saved me having to read 125 comments heading that way!!

Basically a filter had been set on the 'default' or Master and overridden on all the displays - so was there - but wasn't - if you get that. By default the master (or default) is not visible in the list of views

"go to admin/structure/views/settings and check "Always show the master display", after that go to the master display of the "problematic" views and see if there is a unwanted "unknown" filter, if there is remove it."

I will set that as a default from now on - phew!