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

[manuelzomer]

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