There are some coding standard warnings and errors:

phpcs: error
error - Class file names should be based on the class name with "class-" prepended. Expected class-wpmeetuplistwidget.php, but found wpmeetup-widget.php.
phpcs: error
error - When a multi-item array uses associative keys, each value should start on a new line.
phpcs: warning
warning - Array double arrow not aligned correctly; expected 23 space(s) between "'https://wpkoblenz.de'" and double arrow, but found 1.
phpcs: warning
warning - Using the WPCS native whitelist comments is deprecated. Please use the PHPCS native "phpcs:ignore Standard.Category.SniffName.ErrorCode" annotations instead. Found: // WPCS: XSS okay.
phpcs: error
error - Only one argument is allowed per line in a multi-line function call
phpcs: error
error - Closing parenthesis of a multi-line function call must be on a line by itself
phpcs: error
error - Visibility must be declared on method "update"
phpcs: warning
warning - Equals sign not aligned with surrounding assignments; expected 6 spaces but found 1 space