Webitems don't work when in the system.user/user-content location and targeting a dialog

Issue Summary

Webitems don't work when in the system.user/user-content location and targeting a dialog. Also see https://community.developer.atlassian.com/t/web-item-target-type-as-dialog-is-not-working-from-user-profile-menu/44565.

Steps to Reproduce

1. Create an app that declares a dialog and a web item as per the excerpt below.
2. Install the app.
3. Click on the avatar to open the user menu to reveal the "My webitem" menu item.
4. Click on the "My webitem" menu item.

Descriptor excerpt:

"dialogs": [{
  "key": "my-dialog",
  "url": "/etc"
}],
"webitems": [{
  "key": "my-webitem",
  "name" {"value": "My webitem"},
  "location: "system.user/user-content",
  "target": {
    "type": "dialogmodule",
    "options": {
      "key": "my-dialog"
    }
  }
}]

Note that targeting dialog (rather than dialogmodule) also does not work.

Expected Results

At step 4, the dialog appears.

Actual Results

At step 4, the dialog does not appear.

Workaround

None