Unable to add new project task `Cannot read properties of null (reading 'subscribed_to')`

To add a new project task you navigate to the project in question, go to the tasks tab and click the Add button.

Details

After clicking on the add button, the following error is displayed:

Error
Message
Cannot read properties of null (reading 'subscribed_to')

Trace
TypeError: Cannot read properties of null (reading 'subscribed_to')
    at http://127.0.0.1:3000/bundle.js:4179:13
    at mountMemo (http://127.0.0.1:3000/vendors.js:34993:23)
    at Object.useMemo (http://127.0.0.1:3000/vendors.js:52432:18)
    at http://127.0.0.1:3000/vendors.js:78097:34
    at DualFieldSelector (http://127.0.0.1:3000/bundle.js:4166:67)
    at Object.react_stack_bottom_frame (http://127.0.0.1:3000/vendors.js:52120:20)
    at renderWithHooks (http://127.0.0.1:3000/vendors.js:33878:22)
    at updateFunctionComponent (http://127.0.0.1:3000/vendors.js:36382:19)
    at beginWork (http://127.0.0.1:3000/vendors.js:37994:18)
    at runWithFiberInDEV (http://127.0.0.1:3000/vendors.js:27088:30)

Console log shows:

- Page Loader Request {method: 'GET', url: 'http://127.0.0.1:3000/project_management/project/5', headers: Headers, destination: '', referrer: 'about:client', …}


- apiFetch called: GET /project_management/project/5


- react-dom-client.development.js:9410 TypeError: Cannot read properties of null (reading 'subscribed_to')
        at DualFieldSelector.jsx:94:1
        at mountMemo (react-dom-client.development.js:8777:1)
        at Object.useMemo (react-dom-client.development.js:26216:1)
        at exports.useMemo (react.development.js:1251:1)
        at DualFieldSelector (DualFieldSelector.jsx:80:1)
        at Object.react_stack_bottom_frame (react-dom-client.development.js:25904:1)
        at renderWithHooks (react-dom-client.development.js:7662:1)
        at updateFunctionComponent (react-dom-client.development.js:10166:1)
        at beginWork (react-dom-client.development.js:11778:1)
        at runWithFiberInDEV (react-dom-client.development.js:871:1)

    The above error occurred in the <DualFieldSelector> component.

    React will try to recreate this component tree from scratch using the error boundary you provided, RenderErrorBoundary.

Tasks

• Investigate and correct
• Test
• Confirm effected versions

Start with the project task Add flow and inspect DualFieldSelector.jsx around lines 80-94, where the null value is read from subscribed_to. Reproduce the error after loading /project_management/project/5, then trace the data returned by that request. Done means the add-task interface renders without the TypeError and the behavior is covered by a regression test, with affected versions confirmed.