Add getOverlay method to access sketch and selection layers

src/ol/interaction/Select.js

@@ -255,11 +255,13 @@ class Select extends Interaction {
      */
     this.featureLayerAssociation_ = {};
 
-    const features = this.featureOverlay_.getSource().getFeaturesCollection();
-    listen(features, CollectionEventType.ADD,
-      this.addFeature_, this);
-    listen(features, CollectionEventType.REMOVE,
-      this.removeFeature_, this);
+    if (this.featureOverlay_) {
+      const features = this.featureOverlay_.getSource().getFeaturesCollection();
+      listen(features, CollectionEventType.ADD,
+        this.addFeature_, this);
+      listen(features, CollectionEventType.REMOVE,
+        this.removeFeature_, this);
+    }
 
   }
 
@@ -307,6 +309,15 @@ class Select extends Interaction {
     );
   }
 
+  /**
+   * Get the overlay layer that this interaction renders selected features to.
+   * @return {module:ol/layer/Vector} Overlay layer.
+   * @api
+   */
+  getOverlay() {
+    return this.featureOverlay_;
+  }
+
   /**
    * Hit-detection tolerance. Pixels inside the radius around the given position
    * will be checked for features. This only works for the canvas renderer and