Running the Moodle codechecker against your code I found the following error in blocks/quickmail/tests/unit/traits/submits_compose_message_form.php
#88: ····························$$containername[]·=·$recipienttype·.·'_'·.·$id;
Indirect access to variables, properties and methods will be evaluated strictly in left-to-right order since PHP 7.0. Use curly braces to remove ambiguity. (PHPCompatibility.Variables.NewUniformVariableSyntax.Found)
Running the Moodle codechecker against your code I found the following error in blocks/quickmail/tests/unit/traits/submits_compose_message_form.php
I believe the change required is: