Fix a crash that could occur when cancelling an export (fixes #2192)

Source/SPExportController.m

@@ -72,6 +72,9 @@ - (void)_toggleExportButtonWithBool:(NSNumber *)enable;
- (void)_resizeWindowForCustomFilenameViewByHeightDelta:(NSInteger)delta;
- (void)_resizeWindowForAdvancedOptionsViewByHeightDelta:(NSInteger)delta;

- (void)_waitUntilQueueIsEmpty:(id)sender;
- (void)_queueIsEmpty:(id)sender;

@end

@implementation SPExportController
@@ -364,8 +367,20 @@ - (IBAction)cancelExport:(id)sender
	[sender setEnabled:NO];
	
	// Cancel all of the currently running operations
	[operationQueue cancelAllOperations];
	
	[operationQueue cancelAllOperations]; // async call
	[NSThread detachNewThreadWithName:SPCtxt(@"SPExportController cancelExport: waiting for empty queue", tableDocumentInstance) target:self selector:@selector(_waitUntilQueueIsEmpty:) object:sender];
}

- (void)_waitUntilQueueIsEmpty:(id)sender
{
	[sender retain];
	[operationQueue waitUntilAllOperationsAreFinished];
	[self performSelectorOnMainThread:@selector(_queueIsEmpty:) withObject:sender waitUntilDone:NO];
	[sender release];
}

- (void)_queueIsEmpty:(id)sender
{
	// Loop the cached export file paths and remove them from disk if they exist
	for (SPExportFile *file in exportFiles)
	{