fix: unmodifiable error

[fengwk]

Do not write when the buffer cannot be modified. A common scenario is that inputting new questions before the last question has been answered will generate a large amount of error information.

[joeldrotleff]

Works great for me! I notice this error anytime a prompt is being answered by gpt and I navigate to the "toggle sessions" window, and it can't be dismissed (just keeps generating new errors). I can't repro the bug with this fix - nice work