fix: prevent double loading spinners in Form component

[prem22k]

Link to Jira Ticket

Fixes #590

Description

What changed?

I updated the useEffect hook in src/components/Form.tsx to include a proper cancellation pattern.
Specifically:

1. Added a local ignore flag to track if the effect has been cleaned up (unmounted).
2. Updated the cleanup function to set ignore = true.
3. Added a check after the asynchronous createWebformInstance call to immediately destroy the instance if ignore is true.

Why have you chosen this solution?

The previous implementation relied on a ref (isMounted) that could be inconsistent during the rapid mount/unmount cycles triggered by React Strict Mode or fast user navigation. This race condition caused two Formio instances to be created and attached to the DOM, resulting in "double loading spinners" and potential memory leaks.

The new pattern ensures that if the component is unmounted while the form is still initializing, the "zombie" instance is destroyed immediately upon creation, preventing it from ever attaching to the DOM or state. This aligns with the pattern already successfully used in FormBuilder.tsx.

Breaking Changes / Backwards Compatibility

None. This is an internal fix to the component's lifecycle management.

Dependencies

None.

How has this PR been tested?

• Ran npm test locally, and verified that existing tests pass (src/components/__tests__/Form.test.tsx).
• Validated the fix logic by comparing it against the stable implementation in FormBuilder.tsx.

Checklist:

• I have completed the above PR template
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• My changes include tests that prove my fix is effective (or that my feature works as intended)
• New and existing unit/integration tests pass locally with my changes
• Any dependent changes have corresponding PRs that are listed above

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