Keep changing the notes folder from crashing the notes list

[dangduc]

Problem

Changing the notes folder in Preferences crashed the app with EXC_BAD_ACCESS at address 0x0.

#0  -[NotesTableView viewingLocation]                          NotesTableView.m:514
#1  -[AppController notationListMightChange:]                  AppController.m:1804
#2  -[NVBrowserSession refreshKeepingCurrentNote:]             NVBrowserSession.m:463
#3  -[AppController attachLibrary:finishingOldLibrary:]        AppController_MultipleWindows.m
#4  -[NVApplicationController setLibrary:finishingOldLibrary:] NVApplicationController.m:247
#5  -[AppController settingChangedForSelectorString:]          AppController.m:830

Cause

-viewingLocation bounded its lookup with the table's cached row count and then indexed the data source's backing array with it:

if (pivotRow < nRows) {                                  // nRows = [self numberOfRows]
    ... [(FastListDataSource*)[self dataSource] immutableObjects][pivotRow]

Changing the notes folder replaces the library under every open browser window. -attachLibrary:finishingOldLibrary: installs a fresh data source, and the refresh runs while the table's cached count still describes the previous library.

The NULL is legitimate rather than corruption: -[FastListDataSource fillArrayFromArray:] only allocates objects when count > oldArraySize, so a data source that has never held a row keeps it NULL and -immutableObjects returns that.

Measured in lldb at the crash:

Expression	Value
[[self dataSource] immutableObjects]	0x0
[self numberOfRows]	1 (stale)
[[self dataSource] count]	0

So the guard passed on a stale count and dereferenced NULL[0].

This is why direction matters: switching to an empty folder is survivable, while switching back is when a never-populated data source meets a non-zero cached count.

Fix

Bound the lookup by the data source's own count, and skip it when the backing array is absent.

Testing

New suite Tests/Regression/library-switch/, registered in Tests/run-regression-tests.py (14 checks):

• a data source that has never held a row reports zero rows and a NULL backing array
• changing the notes folder installs a different library, and an empty folder opens as empty
• switching back to the original folder restores its notes without crashing

Reverting the guard and rebuilding crashes the suite (exit 245, SIGSEGV) at the switch back, so the check cannot pass vacuously.

The stale-count window cannot be staged directly, because -setDataSource: invalidates the table's cached row count — I tried, and the synthetic precondition failed to hold. It opens only during a real library switch, so the suite drives the switch end to end through the setAliasDataForDefaultDirectory:sender: default that the Preferences "Other..." item writes.

Ran on macOS 13.7.8, Intel Development build. Independent of #33.

https://claude.ai/code/session_01W5dVNP811hKcLKUmJHD1FG