addressed friedbunny's comments

Author: jmkiley

Examples/ObjectiveC/ExtrusionsExample.m

@@ -35,7 +35,7 @@ - (void)viewDidLoad {
 
 - (void)mapView:(MGLMapView *)mapView didFinishLoadingStyle:(MGLStyle *)style {
 
-    // Access the Mapbox Streets source and use it to create a `MGLFillExtrusionLayer`.
+    // Access the Mapbox Streets source and use it to create a `MGLFillExtrusionStyleLayer`. The source identifier is `composite`. Use the `sources` property on a style to verify source identifiers.
     MGLSource *source = [style sourceWithIdentifier:@"composite"];
     MGLFillExtrusionStyleLayer *layer = [[MGLFillExtrusionStyleLayer alloc] initWithIdentifier:@"buildings" source:source];
     layer.sourceLayerIdentifier = @"building";
@@ -48,7 +48,7 @@ - (void)mapView:(MGLMapView *)mapView didFinishLoadingStyle:(MGLStyle *)style {
     layer.fillExtrusionOpacity = [MGLStyleValue valueWithRawValue:@0.75];
     layer.fillExtrusionColor = [MGLStyleValue valueWithRawValue:[UIColor whiteColor]];
 
-    // Insert the fill extrusion layer below a POI label layer.
+    // Insert the fill extrusion layer below a POI label layer. Use the `layers` property on a style to verify built-in layer identifiers.
     MGLStyleLayer *symbolLayer = [style layerWithIdentifier:@"poi-scalerank3"];
     [style insertLayer:layer belowLayer:symbolLayer];
 }

Examples/Swift/ExtrusionsExample.swift

@@ -19,7 +19,7 @@ class ExtrusionsExample: UIViewController, MGLMapViewDelegate {
 
         // Center the map on the Colosseum in Rome, Italy.
         mapView.setCenter(CLLocationCoordinate2D(latitude: 41.8902, longitude: 12.4922), animated: false)
-
+        
         // Set the map view camera's pitch and distance.
         mapView.camera = MGLMapCamera(lookingAtCenter: mapView.centerCoordinate, fromDistance: 600, pitch: 60, heading: 0)
         mapView.delegate = self
@@ -29,22 +29,25 @@ class ExtrusionsExample: UIViewController, MGLMapViewDelegate {
 
     func mapView(_ mapView: MGLMapView, didFinishLoading style: MGLStyle) {
 
-        // Access the Mapbox Streets source and use it to create a `MGLFillExtrusionLayer`.
-        let source = style.source(withIdentifier: "composite")
-        let layer = MGLFillExtrusionStyleLayer(identifier: "buildings", source: source!)
-        layer.sourceLayerIdentifier = "building"
-        
-        // Filter out buildings that should not extrude.
-        layer.predicate = NSPredicate(format: "extrude != false AND height >=0")
-        
-        // Set the fill extrusion height to the value for the building height attribute.
-        layer.fillExtrusionHeight = MGLStyleValue(interpolationMode: .identity, sourceStops: nil, attributeName: "height", options: nil)
-        layer.fillExtrusionOpacity = MGLStyleValue(rawValue: 0.75)
-        layer.fillExtrusionColor = MGLStyleValue(rawValue: .white)
-        
-        // Insert the fill extrusion layer below a POI label layer.
-        let symbolLayer = style.layer(withIdentifier: "poi-scalerank3")
-        style.insertLayer(layer, below: symbolLayer!)
+        // Access the Mapbox Streets source and use it to create a `MGLFillExtrusionStyleLayer`. The source identifier is `composite`. Use the `sources` property on a style to verify source identifiers.
+        if let source = style.source(withIdentifier: "composite") {
+            let layer = MGLFillExtrusionStyleLayer(identifier: "buildings", source: source)
+            layer.sourceLayerIdentifier = "building"
+            
+            // Filter out buildings that should not extrude.
+            layer.predicate = NSPredicate(format: "extrude != false AND height >=0")
+            
+            // Set the fill extrusion height to the value for the building height attribute.
+            layer.fillExtrusionHeight = MGLStyleValue(interpolationMode: .identity, sourceStops: nil, attributeName: "height", options: nil)
+            layer.fillExtrusionOpacity = MGLStyleValue(rawValue: 0.75)
+            layer.fillExtrusionColor = MGLStyleValue(rawValue: .white)
+            
+            // Insert the fill extrusion layer below a POI label layer. Use the `layers` property on a style to verify built-in layer identifiers.
+            if let symbolLayer = style.layer(withIdentifier: "poi-scalerank3") {
+                style.insertLayer(layer, below: symbolLayer)
+            } else {
+                style.addLayer(layer)
+            }
+        }
     }
-    
 }