Internal error 500 accessing Theme > Customize

Just in case anyone else finds this in the future. The issue was that we had an error/warning in our functions file.

When accessing widgets we noticed the error/warning from the file. Resolved this, then the customizer link provided us with an on page error.

The on page error linked to a reimplementation of a method from within a plugin. The plugin was overriding a core method so we removed it and now works as expected.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)