Fix to prevent a null check from crashing the running node server.

lib/intervalBuilder.js

@@ -97,6 +97,12 @@ IntervalBuilder.prototype.determineInitialState = function(timestamp, callback)
 };
 
 IntervalBuilder.prototype.calculateDuration = function(check, begin, end, callback) {
+  // validate "check"; a null check will crash the server.
+  // Example: a null check will occur when a tag is removed from a check.
+  if (check == null) {
+    return;
+  }
+
   var durationBegin = Math.max(begin, check.firstTested),
     durationEnd = Math.min(end, check.lastTested),
     duration = durationEnd - durationBegin;