Runtime Error: Invalid Memory Address or Nil Pointer Dereference

[4ndr0666]

Issue Description
Summary
Micro encountered a runtime error related to an invalid memory address or nil pointer dereference.

Environment Information
Micro version: 2.0.12
Commit hash: c2cebaa
Compiled on: September 09, 2023
Operating System: Linux 6.5.7-arch1-1
Steps to Reproduce
Literally just opened a file.
Expected Behavior
I expected the file to open without any issues.

Actual Behavior
Micro crashed and provided the following stack trace:

Micro encountered an error: runtime.errorString runtime error: invalid memory address or nil pointer dereference
runtime/panic.go:261 (0x55bd011e9818)
runtime/panic.go:260 (0x55bd011e97e6)
github.com/zyedidia/micro/v2/cmd/micro/micro.go:403 (0x55bd0168b5cc)
github.com/zyedidia/micro/v2/cmd/micro/micro.go:394 (0x55bd0168b3cc)
runtime/internal/atomic/types.go:194 (0x55bd011d50f2)
runtime/asm_amd64.s:1650 (0x55bd01206621)

[JoeKar]

Let me guess:
You used the term command?
If yes, then it might be related to #2912 & #2964 or #2823.

[dmaluka]

The crash was at this line:

	screen.Screen.Fill(' ', config.DefStyle)

Probably screen.Screen was nil.

Might be fixed by #2967.

[JoeKar]

Yep, that's what I assumed too. Unfortunately...

Steps to Reproduce
Literally just opened a file.

...didn't really fit into that story.

[dmaluka]

Perhaps some non-trivial things were happening when opening the file (some Lua code or whatever).

And I can't see any other places in the code where TempFini() may not be accompanied with subsequent TempStart().

Update: actually I do see one more such place now, also in overwriteFile():

		screenb = screen.TempFini()
		// need to start the process now, otherwise when we flush the file
		// contents to its stdin it might hang because the kernel's pipe size
		// is too small to handle the full file contents all at once
		if e := cmd.Start(); e != nil && err == nil {
			return err
		}

If cmd.Start() fails, the screen will remain shut down. Worth fixing too?

[dmaluka]

I've just reproduced this crash: set sucmd option to aaa (a non-existing command instead of sudo) and try to save a file with escalated privileges.

Fix: #2971