PyxelEditor crashes when using Sound Editor and pressing 'A' key
**Describe the bug**
When using the Sound Editor and pressing various Keys. The application crashes with the message: "pyxel error: invalid sound index in 'PlaySound'". 
I specifically tried 'A' first but most other letter keys also fail.
This error also arises when I try to click on the keyboard with the mouse.
Looks like it stems from:
piano_keyboard.py Line 130: pyxel.play(1, pyxel.SOUND_BANK_FOR_SYSTEM)

**To Reproduce**
1. Run "pyxeleditor 'filename' " (I used highPitch)
2. Click on Sound Editor (3rd button)
3. Press 'A'
4. Application crashes
5. Command Line shows "pyxel error: invalid sound index in 'PlaySound'"

**Expected behavior**
Application does not crash.
<Optional> Application plays note

**Screenshots**
If applicable, add screenshots to help explain your problem.

**Desktop (please complete the following information):**
 - OS: Windows 10
- Python: 3.7.4

**Additional context**
I would like to help contribute to this problem.
