You are trying to create a custom batch task in Trados Studio, for example from the Projects view, via Batch Tasks > Custom > Task Sequences > Add or under File > Options > Default Task Sequence > Task Sequences > Add, but after you create your batch task and close the Task Sequences window, Trados Studio crashes with the following error:

Value cannot be null.
Parameter name: templates

The issue only occurs when the Custom task sequence is selected when you click on Task Sequences, as can be seen below. Select any of the default task sequences from the drop-down list, and you can add your custom sequence without any errors.

The issue was logged for RWS Development, to implement a fix in a future Trados Studio update.