'Cannot convert undefined or null to object' when rendering field conditionally with React.StrictMode about form HOT 5 CLOSED

Wow! Great catch - we'll tackle this ASAP

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This required a lot of investigation and internal discussion - thanks for your patience everyone!

Should now be fixed in TanStack Form 0.13.7 🎉

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Yes, it took some time to find the problem 😀
Thank you for your support!

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I think it has something to do with issue #495 . The crash point is also
state.meta.errors = Object.values(state.meta.errorMap).filter( (val) => val !== void 0 ); because errorMap is not defined.

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Original comment:

I don't know if I add anything valuable here, but to me, it looks like a new field instance (in form.getFieldInfo('firstName').instances) is created every time the firstName field is rendered, but deleted only every second time it's destroyed. I suspect that this is because the 1st runs are always triggered by <StrictMode> and there's a piece of logic in the useIsomorphicEffectOnce() hook, that prevents the cleanup function from being executed for "dummy React cycles" (this hook is used in useField()):

I think this is what leads to the number of Error messages growing every time we toggle the visibility of the firstName field in the CodeSandbox from the description:

Locally I tried to remove the redundant field instances manually in the packages/form-core/src/FieldApi.ts file (lines 402-414), and it seemed to solve the issue, so I could be on the right track here:

    return () => {
      const preserveValue = this.options.preserveValue
      unsubscribe()
      if (!preserveValue) {
        delete info.instances[this.uid - 1] <-- this is the new line (NOT a real solution)
        delete info.instances[this.uid]
        this.form.deleteField(this.name)
      }


      if (!Object.keys(info.instances).length && !preserveValue) {
        delete this.form.fieldInfo[this.name]
      }
    }

Honestly, I'm still very new to the internals of tanstack/form, so I'm just relying on my instincts here... @crutchcorn Does this make any sense? If yes, should I pick it up?

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