add fix for null pointer exception in _next_fn (issue 518)

lib/index.js

@@ -770,14 +770,16 @@ function Stream(generator) {
     };
 
     this._next_fn = function (xs) {
-        // console.log(self.id, '_next', xs, self.paused);
+        if (self._explicitly_destroyed) {
+            return;
+        }
 
         // It's possible to get into a situation where a call to next() is
         // scheduled asynchonously, but before it is run, destroy() is called,
         // usually by a downstream consumer like take(1). The call to next()
         // still completes, and there is nothing the original caller can do can
         // do. We do not want to throw in that situation.
-        if (self._nil_pushed && !self._explicitly_destroyed) {
+        if (self._nil_pushed) {
             throw new Error('Cannot call next after nil');
         }
 
@@ -1227,7 +1229,11 @@ addMethod('destroy', function () {
 });
 
 Stream.prototype._writeOutgoing = function _writeOutgoing(token) {
-    if (this._nil_pushed && !this._explicitly_destroyed) {
+    if (this._explicitly_destroyed) {
+        return;
+    }
+
+    if (this._nil_pushed) {
         throw new Error('Cannot write to stream after nil');
     }
 

test/test.js

@@ -1040,6 +1040,30 @@ exports.GeneratorStream = {
         test.ok(push.called, 'The push function should have been called.');
         test.ok(!push.threw(), 'The push function call should not have thrown.');
         test.done();
+    },
+    'Cannot read property __ConsumeGenerator__ of null (#518)': function (test) {
+        var clock = sinon.useFakeTimers();
+
+        var s = _(function (push, next) {
+            setTimeout(function () {
+                push(new Error('error'));
+                _.setImmediate(next); // next2
+                next(); // next1
+            }, 0);
+        });
+
+        s.pull(function (err, x) {
+            s.pull(function () {
+            });
+
+            _.setImmediate(function () {
+                s.destroy();
+            });
+        });
+
+        clock.tick(100);
+        clock.restore();
+        test.done();
     }
 };