Do not crash when no parent window was found
Do not crash when no parent window was found

As this turns out to be more easily triggered than originallay
anticipated:
>> test: make gob! [flags: [no-title no-border on-top]]
>> view/as-is/no-wait test
REBOL Host Failure:
Parent window can't be found

However, the result might not be as expected.

Pointillistic/rebol-lang,zsx/r3,Pointillistic/rebol-lang,zsx/r3,Pointillistic/rebol-lang,zsx/r3,Pointillistic/rebol-lang,zsx/r3