[vector_graphics]Add Null Check for Image Retrieval in listener.dart's onDrawImage Function

[wyqlxf]

This PR adds error handling for #168364

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under[^1].
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under[^1].
• All existing and new tests are passing.

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.If you believe this PR qualifies for a test exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!). The test exemption team is a small volunteer group, so all reviewers should feel empowered to ask for tests, without delegating that responsibility entirely to the test exemption group.

[wyqlxf]

This change is exempt from additional testing as it does not alter any existing functionality.

[stuartmorgan-g]

Thanks for the submission!

In the future, please do not delete things from the checklist that is in the PR template; they are there for a reason. This PR is missing required elements described in the checklist (I’ve restored the missing parts to the PR description), which need to be addressed before it moves forward with review.

I am marking the PR as a Draft. Please review the checklist, updating the PR as appropriate, and when that’s complete please feel free to mark the PR as ready for review.

This change is exempt from additional testing as it does not alter any existing functionality.

Could you quote the part of the linked documentation about the test requirement that led you to believe that bugfixes are exempt from testing?

[wyqlxf]

@stuartmorgan-g
I have reviewed the checklist and made the necessary updates as requested, including the addition of the required unit tests to ensure compliance with the testing standards.