Undefined $select variable when upgrading

A. Create a new Moodle 3.4.x site Login as admin. Check that "Enable messaging system (messaging)" is enabled. Create 1 user (admin2). Access to "Site administration > Users > Permissions > Site administrators" and add "admin2" as admin. Login as admin2. From "Site administration > Users > Accounts > Bulk user actions", add admin and admin2 to the selection, choose "Send a message", write any text on the message body (for instance " This is a bulk message sent by the ADMIN2 "), click over the "Save changes" button and confirm you want to send the message to all these users. Logout. Testing scenario: upgrade to master Upgrade the code from 3.4. to master. Login as admin and follow the upgrading steps. CHECK no error is displayed during the upgrade. Open your favourite database client (for instance, adminer) and access to the database where this Moodle instance is installed (for instance, integration_master). CHECK the table mdl_message_read is empty.