Overridden close closes before checking if already closed

The close method in Managed has a check for whether _ptr is zero, and throws an error if so. But if the method is overridden, e.g.

JWM/shared/java/LayerGL.java

Lines 59 to 64 in b3f2289

    
           @Override 
        
           public void close() { 
        
               assert _onUIThread(); 
        
               _nClose(); 
        
               super.close(); 
        
           }

(this kind of thing happens in most overridden close methods) then the native close method will be called before that, resulting in a segfault, which is a lot less clear on what has happened.

This is a pretty minor thing all things considered, as it only affects code that's already incorrect, but I think it's still worth fixing.

Maybe replace assert _onUIThread(); with assert _onUIThread() && !isClosed();?

Yeah that makes sense. PR?

	@Override
	public void close() {
	assert _onUIThread();
	_nClose();
	super.close();
	}