So without seeing your model it would be hard to say why exactly this error is firing. The "Involved 2" variable for a Load tasksequence is the object being loaded from, or the container of the item you're loading. So for whatever reason, in your model you're object to load from is returning as NULL.
If you want to use the profiletasksequence() command. You can open up your script window after the error is thrown, and type the following:
profiletasksequence(gettasksequence(node("Crane1", model()),0));
Just replace the "Crane1" with whatever the name of your Crane is. The gettasksequence() first asks for a reference to the dispatcher of the task sequence (in this case it is your crane itself) and then the rank of the task sequence. The output of the profiletasksequence function will be displayed in your model's Output Console.