Fix usage of WeakPtr valid check

[scutlight]

Description

Some codes miss use the WeakPtr API to check its validity. The API get() of WeakPtr just for returning the internal pointer, without checking the pointer's validity. To correct it, just use WeakPtr's operator bool().

Related Issues

none

Tests

I added the following tests:

Replace this with a list of the tests that you added as part of this PR. A
change in behaviour with no test covering it will likely get reverted
accidentally sooner or later. PRs must include tests for all
changed/updated/fixed behaviors. See testing the engine for instructions on
writing and running engine tests.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the contributor guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the C++, Objective-C, Java style guides for the engine.
• I read the tree hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation.
• All existing and new tests are passing.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.
  • I wrote a design doc: https://flutter.dev/go/template Replace this with a link to your design doc's short link
  • I got input from the developer relations team, specifically from: Replace with the names of who gave advice
  • I wrote a migration guide: https://flutter.dev/go/breaking-changes-template Replace this with a link to a pull request that adds the migration guide to https://flutter.dev/docs/release/breaking-changes

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[cbracken]

This LGTM in principle, but would like to get @blasten's thoughts with respect to dynamic thread merge/de-merging. If that scenario would be problematic for this patch we should probably add a test that would trigger this, and add comments where we're bypassing WeakPtr's thread-checking.

[cbracken]

Thanks for the cleanup @scutlight!