readthefuckingmanual.net

You're kidding, right?

This can be found in many applications, but more likely one you\'re writing yourself (shame on you!)

Here's a description of it from the first search result:
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Thrown when an application attempts to use null in a case where an object is required. These include:

    * Calling the instance method of a null object.
    * Accessing or modifying the field of a null object.
    * Taking the length of null as if it were an array.
    * Accessing or modifying the slots of null as if it were an array.
    * Throwing null as if it were a Throwable value. 
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A summary of the above:
You are probably trying to use something that hasn\'t been initialized.

The solution:
Make sure the object, method, parameter, whatever is initialized before you use it!

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