I have a situation where in some cases, the parameters to the form (to simplify, let's say the form is 'delete_thingy') are invalid. This most often occurs if somebody else has already deleted the thingy, between when the user loaded the page listing thingies and when they clicked to delete it. In its definition, the form ends up calling something like $thingy = thingy::get($thingyid) where $thingyid is a form parameter. This throws an exception because the thingy no longer exists. I would like to show a nicer error in this case. Here is the workaround I am using, in the form: protected function definition() { // ... try { // ...all the existing definition, some of which (at start) throws exception... } catch (\coding_exception $e) { // If there is an error finding data required, we show a 'loadingerror' element, which // is used by JavaScript to detect the error and automatically cancel the form. $mform->addElement('static', 'loadingerror', '', '<div hidden>' . s($e->getMessage()) . '</div>'); } } And in the JavaScript just after showing the modal form: // This event happens when the form has actually loaded. It is a jQuery event and doesn't get // sent natively. jQuery(document.body).one(ModalEvents.bodyRendered, async() => { // Check if the form contains an error. We can't handle exceptions if they occur during // form loading, so the form adds a special item instead of throwing an exception, and // we can then show it as a modal. if (modalForm.getFormNode().querySelector('div[id^=fitem_id_loadingerror]')) { modalForm.modal.destroy(); const modal = await ModalFactory.create({ type: ModalFactory.types.ALERT, title: getString(...), body: getString(...), removeOnClose: true, }); modal.show(); modal.getRoot().on(ModalEvents.hidden, () => { location.reload(); }); } }); You might be wondering why the event listens on document.body, this is obviously rubbish but without doing that you actually need another event listener ModalForm.events.LOADED so that the modal exists (I'm not sure you can even get at the modal from there anyway, but mabe you can) but I couldn't be bothered with nested events. This gives the same user experience you'd get if the ERROR event works (popup form starts to appear, then goes away and you get an error modal instead) but the code is a bit ick.

The attached mdl80608.zip is a local plugin. Expand the zip and drop the mdl80608 directory inside 'local' in your Moodle installation, then visit admin page to install. Visit [Your moodle site url] /local/mdl80608/ to test. There should be 3 buttons. Click the 'submission exception' button. Click on 'Save changes'. Close the error popup that appears. Confirm that 'Cancel' and 'Save changes' buttons are enabled (these would be disabled pre-patch). Close all popups. Click the 'definition exception' button. Confirm you do not see Moodle's default exception popup, and instead see a popup with the contents 'This would usualbly be a nice error message...Exception during definition'.