Fix cannot read property 'stdin' of null in some cases

Add check for phantom._isDisposed before scheduling a new ping. This prevents a new ping from being scheduled
when a pong message is received after phantom.dispose() has been called.

lib/Phantom.js

@@ -306,6 +306,12 @@ Phantom.prototype._schedulePing = function () {
         // There is already a ping scheduled. It's unnecessary to schedule another one.
         return;
     }
+    if (this._isDisposed) {
+        // No need to schedule a ping, this instance is about to be disposed.
+        // Catches rare edge cases where a pong message is received right after the instance has been disposed.
+        // @see https://github.com/peerigon/phridge/issues/41
+        return;
+    }
     this._pingTimeoutId = setTimeout(this._write, pingInterval, { action: "ping" });
 };
 

test/Phantom.test.js

@@ -458,6 +458,17 @@ describe("Phantom", function () {
                     });
             });
 
+            // @see https://github.com/peerigon/phridge/issues/41
+            it("should not schedule a new ping when a pong message is received right after calling dispose()", function () {
+                var message = JSON.stringify({ status: "pong" });
+                var promise = phantom.dispose();
+
+                // Simulate a pong message from PhantomJS
+                phantom._receive(message);
+
+                return promise;
+            });
+
         });
 
     });