Crash on paste after running batch macro

[SteveDaulton]

Describe the bug
When there is audio on the Audacity clipboard, attempting to paste after running a batch processing macro causes Audacity to crash.

To Reproduce
Steps to reproduce the behavior:

1. Copy some audio to the clipboard
2. Clear all tracks from the project
3. Run a macro on one or more files (example, "MP3 Conversion") and wait for the batch to complete.
4. "Edit menu > Paste"
5. Observe crash

Expected behavior
The copied audio to be pasted.

Screenshots

Additional information (please complete the following information):

• OS: Xubuntu 20.04 / first reported with Windows 10.
• Version: Confirmed with Audacity 3.0.2 / 3.0.3 / 3.0.5 / 3.1.0

Additional context
The original report on the forum was due to a user attempting to use a macro that they were previously using (successfully) in Audacity 2.4.2, in which they would copy audio to the clipboard, then use a macro to append the audio to multiple files:

SelectAll:
Normalize:ApplyGain="1" PeakLevel="-0.1" RemoveDcOffset="1" StereoIndependent="0"
CursTrackEnd:
Paste:
ExportMp3:

While this did not crash, the macro failed to append the audio from the clipboard. The user has reverted to Audacity 2.4.2.

[SteveDaulton]

Assert:
UndoManager.cpp(384): assert "n < stack.size()" failed in SetStateTo().

BACKTRACE:
[1] UndoManager::SetStateTo(unsigned int, std::function<void (UndoStackElem const&)> const&)
[2] ProjectHistory::SetStateTo(unsigned int, bool)
[3] ProjectHistory::RollbackState()
[4] wxEvtHandler::TryHereOnly(wxEvent&)
[5] wxEvtHandler::TryBeforeAndHere(wxEvent&)
[6] wxEvtHandler::ProcessEventLocally(wxEvent&)
[7] wxEvtHandler::ProcessEvent(wxEvent&)
[8] wxEvtHandler::ProcessPendingEvents()
[9] wxAppConsoleBase::ProcessPendingEvents()
[10] wxApp::DoIdle()
[11] g_main_context_dispatch
[12] g_main_loop_run
[13] gtk_main
[14] wxGUIEventLoop::DoRun()
[15] wxEventLoopBase::Run()
[16] wxAppConsoleBase::MainLoop()
[17] wxAppConsoleBase::OnRun()
[18] wxAppBase::OnRun()
[19] AudacityApp::OnRun()
[20] wxEntry(int&, wchar_t**)
[21] wxEntry(int&, char**)
[22] main
[23] __libc_start_main
[24] _start

[crsib]

Oh wow, it's one of the most common crashes in Audacity! Thank you for STR, @SteveDaulton!

[crsib]

Another way:

1. Run Export MP3 on a larger file
2. Cancel
3. Run Export MP3 on a larger file
4. Cancel

Or even

1. Run Export MP3 on a larger file
2. Run Export MP3 on a larger file
3. Cancel

[crsib]

Optionally, apply the macro to a few files. If the macro succeeds on the first file and fails on any later - Audacity crashes

[crsib]

1. Generate sound
2. Copy to clipboard
3. Remove track
4. Use macro on file
5. Paste
6. Undo
7. Use macro on file

Audacity crashes

[crsib]

As for the original issue with Paste not working for the user - the behavior was disabled by @Paul-Licameli: 105558c while fixing https://bugzilla.audacityteam.org/show_bug.cgi?id=2567

Do we want to restore it?

[SteveDaulton]

It's a useful feature (as demonstrated by the fact that this bug came to light). I don't think that useful features should be removed unless they are replaced by a superior alternative.

[crsib]

I have split this issue in two. The issue reported on the forum is moved to #2254