Correcting error during uninstallation of 'Page Lock' module

Correcting error during uninstallation of "Page Lock" module - "ContextErrorException: Notice: Undefined property: Zikula\Module\PageLockModule\PageLockModuleInstaller::$dropManager".

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | -
| License       | MIT
| Doc PR        | -