A form containing an editor throws the moment the page is opened, not on submit, if its app and key do not name a real extension.

The symptom

OutOfBoundsException: Admin
#0 /applications/myapp/modules/admin/.../controller.php(340): IPS\Helpers\Form\Editor->__construct(...)

The exception message is the key alone, with no mention of the app, which makes it read like a missing language string.

The cause

new Editor( 'body', '', TRUE, array( 'app' => 'myapp', 'key' => 'Admin' ) )

This resolves eagerly to applications/myapp/extensions/core/EditorLocations/Admin.php. If that file does not exist, the constructor throws.

The fix

Either register the extension, or point at an existing one. For an admin-authored email or announcement body, core/Admin is what core's own Bulk Mail uses:

new Editor( 'body', $value, TRUE, array(
    'app'         => 'core',
    'key'         => 'Admin',
    'autoSaveKey' => 'myapp-thing-' . $id,
    'tags'        => array( '{member_name}' => 'Member name' ),
) )

The tags option adds an insert menu to the editor toolbar, which is a better home for placeholders than a help box above the field.

Testing it

This fails inside an ACP controller, which cannot be instantiated from the command line. A static check — reading the source for app/key pairs and confirming the file exists — catches it without a browser.

Related: "This theme may be out of date" usually means your form field, not the theme, The core/EditorMedia extension in Invision Community 5