It does not give an error if tuning letter of tab is invalid

[amin45mh]

Description: It doesn’t give an error if any tunning letter of the guitar is wrong
Steps to reproduce: Run the code and choose a guitar tablature with the wrong tuning letter at any line
Expected result: An Error message shows up
Actual result: It converts invalid tab wrong
screenshots:
As you can see the tuning letter of first line is wrong

It works without error although its invalid tab

Version: Gradle Eclipse
Priority: high
Status: Ready for development