Added check for PopperInstance before destroying it

[MathiasWP]

I got this error:

Saw that it was connected to this piece of code:

And if i understand this correctly, then it is valid that this.popover can be undefined when calling disconnectedCallback, because it is asynchronously created.

I therefore added this check in the disconnectedCallback method in the dropdown.ts file.

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[claviska]

Thanks. This is what's happening in tooltip, so makes sense to add it here.