child is already inserted in a tree

Advice on the net is to create a cut down version prior to creating a TSI. Easier said than done, given that this is a large program. And somehow one is supposed to reduce the problem to <250 lines which seems utterly infeasible in this case. Nevertheless, some days and many hours later, the program is vastly reduced, but still there are >150 files to comb through.

The useful outcome of this work however is that I can characterise the 'phenomonology' that surrounds the appearance of these message strings. Sometimes after a rebuild, the cut down program must be launched sequentially several times before the messages begin once again to appear. I suppose therefore that somewhere in the program worker threads are launched that SceneKit cannot manage properly, or that there is something I am doing that prepares the ground for thread conflict. I do not know.

Some days later, the tedium of cutting down a massive project is finally rewarded. The code was using a method of opening a document that perhaps is no longer supported?

// problem somehow associated with this line of code // maybe the GPU is not cleaned out properly??? [self openNewDocumentIfNeeded];

Once that line was commented out, the problem went away.

In case one might spend more time wondering why .... ? ... then here is the offending code segment

// this code segment is from https://stackoverflow.com/questions/7564290/why-isnt-applicationshouldopenuntitledfile-being-called

• (void) openNewDocumentIfNeeded {

  // Get the recent documents

  NSDocumentController * controller = [NSDocumentController sharedDocumentController];

  NSArray * documents = [controller recentDocumentURLs];

  // how many documents are in the recent list

  // .. the most recent document added is at the bottom of the list

  // ... but somehow I've noticed that the order of documents in the list can change

  int nRecents = (int)[documents count];

  // If there is a recent document, try to open it.

  if ( nRecents > 0 ) {

    NSError * error = nil;
  
    NSURL * url = [documents objectAtIndex:(nRecents-1)];
  
    [[NSDocumentController sharedDocumentController] openDocumentWithContentsOfURL:url display:YES completionHandler:(void (^)(NSDocument * document, BOOL documentWasAlreadyOpen, NSError * error ) ) ^{
  
        ;
    }];
    
    // If there was no error, then prevent untitled from appearing.
    if ( error == nil ) {
        if ( nRecents > 0 ) {
            return;
        }
    }
    
  

  }

  // Open an untitled document what if there is no document. (restored, opened).

  if ( nRecents == 0 ){

    [[NSDocumentController sharedDocumentController] openUntitledDocumentAndDisplay:YES error: nil];
  

  }

}