Fix to check if block still exists

src/engine/sequencer.js

@@ -97,7 +97,7 @@ Sequencer.prototype.stepThread = function (thread) {
     // If the primitive would like to do control flow,
     // it can overwrite nextBlock.
     var currentBlock = thread.nextBlock;
-    if (!currentBlock) {
+    if (!currentBlock || !this.runtime.blocks[currentBlock]) {
         thread.status = Thread.STATUS_DONE;
         return;
     }

vm.js

@@ -1662,7 +1662,7 @@
 	    // If the primitive would like to do control flow,
 	    // it can overwrite nextBlock.
 	    var currentBlock = thread.nextBlock;
-	    if (!currentBlock) {
+	    if (!currentBlock || !this.runtime.blocks[currentBlock]) {
 	        thread.status = Thread.STATUS_DONE;
 	        return;
 	    }