You are trying to use something that is null (or Nothing in VB.NET). This means you either set it to null, or you never set it to anything at all.
Like anything else, null gets passed around. If it is null in method "A", it could be that method "B" passed a null to method "A".
The rest of this article goes into more detail and shows mistakes that many programmers often make which can lead to a NullReferenceException.