r/appstream_user_stack_association: prevent panic during resource read

[anGie44]

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Closes #22468

Output from acceptance testing:

--- PASS: TestAccAppStreamUserStackAssociation_disappears (24.42s)
--- PASS: TestAccAppStreamUserStackAssociation_basic (28.31s)
--- PASS: TestAccAppStreamUserStackAssociation_complete (45.05s)

[ewbankkit]

LGTM 🚀.

% make testacc TESTS=TestAccAppStreamUserStackAssociation_ PKG=appstream ACCTEST_PARALLELISM=2
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./internal/service/appstream/... -v -count 1 -parallel 2 -run='TestAccAppStreamUserStackAssociation_'  -timeout 180m
=== RUN   TestAccAppStreamUserStackAssociation_basic
=== PAUSE TestAccAppStreamUserStackAssociation_basic
=== RUN   TestAccAppStreamUserStackAssociation_disappears
=== PAUSE TestAccAppStreamUserStackAssociation_disappears
=== RUN   TestAccAppStreamUserStackAssociation_complete
=== PAUSE TestAccAppStreamUserStackAssociation_complete
=== CONT  TestAccAppStreamUserStackAssociation_basic
=== CONT  TestAccAppStreamUserStackAssociation_complete
--- PASS: TestAccAppStreamUserStackAssociation_basic (21.84s)
=== CONT  TestAccAppStreamUserStackAssociation_disappears
--- PASS: TestAccAppStreamUserStackAssociation_complete (39.81s)
--- PASS: TestAccAppStreamUserStackAssociation_disappears (18.98s)
PASS
ok  	github.com/hashicorp/terraform-provider-aws/internal/service/appstream	44.468s

[github-actions]

This functionality has been released in v4.11.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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