WordPress Customizer Selective Refresh: works only on first setting change

Examine the actual value being returned from the checkbox, using var_dump(). It looks like it is not matching true or false, but always falling through to the default case.
Even your default value of “” is not true or false.

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