We guarded one step against a null navigable, but the very next step
also needs to be protected. Let's just abort early instead. This was
caught by the following imported WPT test:
html/dom/elements/the-innertext-and-outertext-properties/innertext-setter.html
This test adds a <frame> element and immediately removes it, but the
task to process the src attribute is already queued. Note that <iframe>
would have the same issue, but this test does not include them.
