[mainloop-] prevent input() errors while pasting keys

[midichef]

When pasting a string into the terminal, a long sequence of keys such as aaaaaaaaaaaO (with a trailing space), will trigger an error:

File "/home/midichef/.local/lib/python3.10/site-packages/visidata/_input.py", line 358, in editText
v = vd.editline(vd.activeSheet._scr, y, x, w, display=display, **kwargs)
File "/home/midichef/.local/lib/python3.10/site-packages/visidata/_input.py", line 288, in editline
ch = vd.getkeystroke(scr)
File "/home/midichef/.local/lib/python3.10/site-packages/visidata/vdobj.py", line 141, in getkeystroke
self.drainPendingKeys(scr)
File "/home/midichef/.local/lib/python3.10/site-packages/visidata/vdobj.py", line 124, in drainPendingKeys
scr.timeout(0)
AttributeError: 'NoneType' object has no attribute 'timeout'

(This is for v3.0 and develop. For v2.11.1, pasting this string causes visidata to freeze and consume a lot of CPU.)

Why this happens:
After a new active sheet is created, there is a brief interval when the sheet does not yet have a _scr value. That interval lasts until the next time draw_all() is called in mainloop(). When handling pasted keys, input() can run in that brief interval, and it expects the active sheet's _scr value to have already been set.

What this PR changes:
After each command in the mainloop, if the active sheet has no screen in _scr, draw_all() will run, to give the sheet a screen.

Other considerations when pasting:
Even after this patch, pasting quick sequences of keys into visidata will not always work well. For example, consider the sequence O (options-global) and Enter. It can create an OptionsSheet and run edit-option before the new Sheet loads any rows, causing a traceback.

To run a series of commands, users should not paste keys into the terminal, unless they have a delay between keys. The reliable way to run a series of commands is to use macros, or replay commands from .vdj files.

[saulpw]

Okay, this makes sense to me. Thanks for digging into the cause of this!