Added precaution to make sure best-effort updaters aren't mid-update during document destruction.

OSBindings/Mac/Clock Signal/Documents/MachineDocument.swift

@@ -92,6 +92,7 @@ class MachineDocument:
 	}
 
 	override func close() {
+		bestEffortUpdater.flush()
 		actionLock.lock()
 		drawLock.lock()
 		openGLView.invalidate()

OSBindings/Mac/Clock Signal/Updater/CSBestEffortUpdater.h

@@ -25,5 +25,6 @@
 @property (nonatomic, weak) id<CSBestEffortUpdaterDelegate> delegate;
 
 - (void)update;
+- (void)flush;
 
 @end

OSBindings/Mac/Clock Signal/Updater/CSBestEffortUpdater.m

@@ -63,4 +63,9 @@
 	}
 }
 
+- (void)flush
+{
+	dispatch_sync(_serialDispatchQueue, ^{});
+}
+
 @end