react-hook-form: TypeError property 0 is non-configurable and can't be deleted - Firefox

Describe the bug When using a file type input in Firefox and unmounting a component, the unsetObject function (src/utils/unset.ts, line 26) tries to delete an element of FileList Array. In Firefox, this object is non-configurable, throwing an error.

To Reproduce Steps to reproduce the behavior:

1. Using Firefox, create a form with an input file and register it in react-hook-form
2. Add a file to it
3. Unmount the component
4. See error

Codesandbox link Will add later

Expected behavior No error.

Screenshots

Desktop (please complete the following information):

• OS: Windows 10, macOS Catalina
• Firefox
• Version 72.0.2 (macOS)

Smartphone (please complete the following information):

• Not tested

Additional context In my app, the input is in a child Materials-UI modal. The input, however, is a native HTML. After clicking submit, handleSubmit execute a function that updates the parent state, closing the modal. Then, the error occurs.

I couldn’t use react-hook-form for this input type, however, I still used it with the other form inputs. As a workaround, I set an onChange in the file input that sets a state with the file list. Then, at submitting, before sending the form data collected by react-hook-form to the API, I injected the state data.