Fix: addEditSheet is not initialized reliably

Fix: in the current state of the tutorial, addEditSheet is not properly/reliably initialized:

1. openPopulateSheet only passes values (or default values) in "edit" mode, but is also called for 'add' mode, the unnecessary if-condition can be removed
2. The initialization of the text fields in addEditSheet via text: mode === "add" ? "" : name does not seem to work reliably for the text property:
   1. Add a Kountdown to the list
   2. Try to add a second element: addEditSheet should be empty but still contains the values of the last added element