Users have the ability to create alert triggers without alerting-method

**Describe the bug**
When creating a trigger of the type alert, it is possible to **not** choose an "Alert me using..." method, this results in a trigger which is active but doesn't do anything.

**To Reproduce**
Steps to reproduce the behavior:
1. Select a sensor
2. Create a new trigger
3. Choose "Alert Trigger" as trigger type
4. Save without choosing anything in "Alert me using..."

**Expected behavior**
The user should not be able to save if no alert method is selected, the method "Visual Trigger" should be pre-selected. This is also the behaviour on Android.

**Screenshots**

Start by reproducing the alert-trigger creation flow: select a sensor, choose Alert Trigger, and save without an alert method. Verify that saving is prevented when no method is selected and that Visual Trigger is pre-selected, matching the described Android behavior.

Written by the indexing model from the issue text.