Problem with cache clearing

There are two problems with the code that invalidates cache.

First, when checking if a module exists, the module tries to look up a service
$module_handler->moduleExists('cache.page')

Then, when the settings form is constructed, there's a check on get('bin') which does not return the value of the variable $bin.

if ($cache_page->get('bin') === 'cache_page')

$cache_page->get('bin') returns FALSE, so the following line is never executed.