Ignore calls to Session.requestAnimationFrame() if the session has been ended

docs/api/session.md

 
 Schedules a call to the `callback` function, which is intended to update and render the virtual scene. Your `callback` function must itself call `session.requestAnimationFrame()` again in order to continue to update and render the virtual scene.
 
-!!! info "Note"
+!!! info "Notes"
 
     `session.requestAnimationFrame()` is analogous to `window.requestAnimationFrame()`, but they are not the same! The former is a call to the WebAR engine, whereas the latter is a standard call to the web browser.
 
+    This call will be ignored and an invalid handle will be returned if the session has been [ended](#ended) (*since 0.3.0*). Previously, it would raise an exception.
+
 **Arguments**
 
 * `callback: function`. A function that receives two parameters:
 **Example**
 
 ```js
+//
+// This is the animation loop:
+//
+
 function animate(time, frame)
 {
     // update and render the virtual scene
     // ...
 
-    // repeat the call
+    // repeat
     session.requestAnimationFrame(animate);
 }
 
+// start the animation loop
 session.requestAnimationFrame(animate);
 ```
 
 
 **Arguments**
 
-* `handle: SessionRequestAnimationFrameHandle`. A handle returned by `session.requestAnimationFrame()`.
+* `handle: SessionRequestAnimationFrameHandle`. A handle returned by `session.requestAnimationFrame()`. If the handle is invalid, this method does nothing.
 
 ### end
 

src/core/session.ts

     {
         const handle: SessionRequestAnimationFrameHandle = Symbol('raf-handle');
 
-        if(this._active)
+        if(this._active) {
             this._rafQueue.push([ handle, callback ]);
-        else
-            throw new IllegalOperationError(`Can't requestAnimationFrame(): session ended.`);
+        }
+        else {
+            // if the session is inactive, we simply ignore this call
+            // this is friendly behavior, since RAF is used in animation loops
+        }
 
         return handle;
     }