sidebar causes javascript error if modal=false

[wittdavid]

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Current behavior
Sidebar - ngOnDestroy() calls destroyModal() which causes error "Failed to execute 'removeChild' on 'Node': parameter 1 is not of type 'Node'".

SPECIFIC CONDITION - input parameter "modal" is set to false. As a result. enableModality() will NOT be called, and therefore "this.mask" will be null.

Expected behavior
No javascript error.

Minimal reproduction of the problem with instructions
Set input parameter "modal" to false. Make sidebar visible. Cause page transition that results in ngOnDestroy() to be called.

Need to check if "this.mask" before attempting to call removeChild().

Broken as part of commit for: #10639

What is the motivation / use case for changing the behavior?

Please tell us about your environment:

• Angular version: 5.X

• PrimeNG version: 5.X
  12.2.1

• Browser: [all | Chrome XX | Firefox XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]

• Language: [all | TypeScript X.X | ES6/7 | ES5]

• Node (for AoT issues): node --version =