Crash after running shell command (Ctrl+B)

Description of the problem or steps to reproduce

I was reading the micro documentation (keybindings) and I decided to run a shell command (Ctrl+B). The command was pwd. After pressing enter to return to micro, it crashed:

pi@emile:/usr/local/bin $ micro
/usr/local/bin


Press enter to continue
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x4c pc=0x20b2fc]

goroutine 12 [running]:
github.com/zyedidia/micro/cmd/micro/vendor/github.com/zyedidia/tcell.(*tScreen).PollEvent(0x0, 0x126697e4, 0x0)
	/home/zbyedidia/gopath/src/github.com/zyedidia/micro/cmd/micro/vendor/github.com/zyedidia/tcell/tscreen.go:801 +0x14
main.main.func5()
	/home/zbyedidia/gopath/src/github.com/zyedidia/micro/cmd/micro/micro.go:454 +0x34
created by main.main
	/home/zbyedidia/gopath/src/github.com/zyedidia/micro/cmd/micro/micro.go:451 +0x1458

I tried to reproduce this bug but I have failed.

Specifications

Version: 1.4.0
Commit hash: af520cf
Compiled on January 26, 2018
OS: Raspbian Stretch (running on a Raspberry Pi 3B)
Terminal: LXTerminal 0.3.0

Start with the panic location in cmd/micro/vendor/github.com/zyedidia/tcell/tscreen.go:801 and the event-handling goroutine in cmd/micro/micro.go around lines 451-454. Reproduce the Ctrl+B shell command flow with pwd on the reported environment if possible, then trace why returning to micro can leave PollEvent without a valid screen. Done means returning from the shell command no longer crashes.