Clearing a p-columnFilter / p-dropdown using "showClear" X-button throws a TypeError

[austindeboer]

[X ] bug report => Search github for a similar issue or PR before submitting
[ ] feature request => Please check if request is not on the roadmap already https://github.com/primefaces/primeng/wiki/Roadmap
[ ] support request => Please do not submit support request here, instead see http://forum.primefaces.org/viewforum.php?f=35

**Current behavior**
Clearing a p-columnFilter (that's paired with a p-dropdown for selecting an option) using the `[showClear]="true"` button creates a `ERROR TypeError: Cannot read property 'classList' of null at Function.hasClass, at ColumnFilter.isOutsideClicked, at primeng-table.js:4999....` etc etc etc.

**Expected behavior**
Clearing a p-columnFilter isomg the `showClear` X button clears the value, but should not throw an error since this is expected.

**Please tell us about your environment:**
Windows 10, Visual Studio 2019, NPM, HTTP server

* **Angular version:** 
11.0.2

* **PrimeNG version:**
^9.1.3

* **Browser:**
All

* **Language:**
Typescript