In GeneratorStart and AsyncGeneratorStart, the provided generatorBody parameter can be an Abstract Closure in which case the LexicalEnvironment of the running Execution Context may not be set.

We should either:

1. Not call DisposeResources when the LexicalEnvironment is not defined (which should only be for Abstract Closures), or
2. Move the DisposeResources call to the Evaluation for FunctionBody, as suggested in this comment.