Invalidate backup timer when removing backup

The backup timer is retaining our document so the document would live for up to two seconds after it was closed, this was mainly a problem for untitled documents, as it meant the next new untitled document created would get a display name of “untitled n”.

Frameworks/document/src/OakDocument.mm

@@ -551,6 +551,9 @@ NSString* OakDocumentBookmarkIdentifier           = @"bookmark";
 
 - (void)removeBackup
 {
+	[_backupTimer invalidate];
+	_backupTimer = nil;
+
 	if(_backupPath)
 	{
 		[[NSFileManager defaultManager] removeItemAtPath:_backupPath error:nullptr];