app.activeWindow.activeLayer = "Michel"; 

try { 
	app.activeWindow.activeLayer = "Bilder"; 
} 
catch (e) { 
	alert ("Die Ebene \"Bilder\" existiert nicht.", "Achtung"); 
	exit(); 
} 

var actLayer = app.activeDocument.layers.itemByName("Bilder"); 
if (actLayer.isValid) { 
	app.activeWindow.activeLayer = actLayer; 
} 
else { 
	alert ("Die Ebene \"Bilder\" existiert nicht.", "Achtung"); 
	exit(); 
}