I have to admit the facts known to me are simply: plugging in the HDMI cable stopped the bug.
We didn’t dig further than that, though the error was quite obscure until we read the source code, at which point we realized that the “FB” the error mentioned meant the Linux console framebuffer.
I assume the Python module for operating this device ships with a GUI, which is couple to some Linux framebuffer initialization code, which in turn only works if the framebuffer is enabled, which presumably it isn’t if the host boots without an HDMI device connected.
I have a Raspberry Pi with a Sense Hat that has never had an HDMI cable plugged into it. I have written small programs with the Python module that operate the LED matrix that work fine AFAICT. The LED matrix also happens to be a framebuffer device, and it is possible that your error was about the LED matrix.
The LED matrix is an RGB565 framebuffer with the id "RPi-Sense FB". The appropriate device node can be written to as a standard file or mmap-ed.
We didn’t dig further than that, though the error was quite obscure until we read the source code, at which point we realized that the “FB” the error mentioned meant the Linux console framebuffer.
I assume the Python module for operating this device ships with a GUI, which is couple to some Linux framebuffer initialization code, which in turn only works if the framebuffer is enabled, which presumably it isn’t if the host boots without an HDMI device connected.