Default mail application setting not honoured by okular

I have set my default mail application to be Thunderbird. Also, I do not have Kmail on my system.

When I click on a mailto: link in a PDF file using okular, I get an error message shown in the attached screenshot. This was not so before, when a Thunderbird compose window would open.

How might I correct this to get the previous behaviour?


P.S. Interestingly, the builtin PDF reader in Chromium opens a Thunderbird compose window as expected.