Fix error handling in onLoadChildren, onLoadItem and onSearch

[dsst95]

Changed error handling for onLoadChildren, onLoadItem and onSearch, since the existing implementation leads to an error according to an deprecated method call. According to the new android documentation for an error a null result must be send and the error state must be set.

• Fixes #1081

Pre-launch Checklist

• [x] I read the CONTRIBUTING.md and followed the process outlined there for submitting PRs.
• [x] My change is not breaking and lands in minor branch OR my change is breaking and lands in major branch.
• [x] If I'm the first to contribute to the next version, I incremented the version number in pubspec.yaml according to the pub versioning philosophy.
• [x] I updated CHANGELOG.md to add a description of the change (format: * DESCRIPTION OF YOUR CHANGE (@your-git-username)).
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I ran dart analyze.
• [x] I ran dart format.
• [x] I ran flutter test and all tests are passing.