When a link exists in a course to an external tool which has been deleted, an attempted launch generates an exception. This is because the lti_get_instance_type method is defined as returning an object (or null), but for a missing tool a boolean value of false will be returned from the call to the lti_get_type method. In Moodle 3.11.4+ (Build: 20211123) the error reported to the user is:

Error code: generalexceptionmessage
Stack trace:
line 521 of /mod/lti/locallib.php: TypeError thrown
line 535 of /mod/lti/locallib.php: call to lti_get_instance_type()
line 714 of /mod/lti/locallib.php: call to lti_get_launch_data()
line 88 of /mod/lti/launch.php: call to lti_launch_tool()