timeout() throws exception when source is synchronous

[kwonoj]

Bug Report

Current Behavior
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Reproduction

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import {of} from 'rxjs';
import {timeout} from 'rxjs/operators';

of(1).pipe(timeout(100)).subscribe((x) => console.log(x));

This raises

Cannot read property 'unsubscribe' of undefined 

now, previously handled.

Expected behavior
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Environment

• Runtime: [e.g. Node v${x}, Chrome v${x}]
• RxJS version:
• (If bug is related) Loader, build configuration: [e.g webpack, angular-cli version, config]

Possible Solution

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