No good response if there be a missing tuning letter

[amin45mh]

Description: It doesn’t give an error if there is a missing tuning letter
Steps to reproduce: Run the code and choose a guitar tablature with missing tuning letter
Expected result: An Error message shows up
Actual result: It converts invalid tab wrong
screenshots:
As can be seen, lines do not have tuning letter

But it converts a wrong tab

Version: Eclipse 2020-12 (4.18.0)
Priority: high
Status: Ready for development