How can use Object.getOwnPropertyNames with this operator TypeScript

Question

class MyClass {
    private firstKey: string | null = 'Test';
    private secondKey: number | null = 123;
    private thirdKey: string[] | null = [ 'first', 'second', 'third' ];

    constructor( firstParam: string, secondParam: string[] ) {}

    destroy() {
        for ( const propertyName of Object.getOwnPropertyNames( this ) ) {
            this[ propertyName ] = null;
        }
    }
}

I got an error on this code this[ propertyName ] and the error message is:

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'MyClass'.
No index signature with a parameter of type 'string' was found on type 'MyClass'.

How can solve this error? Especially that I can't give any type for this operator?