NullPointerException

Introduction

NullPointerException is a Scala runtime reported error which can happen when a variable is accessed before it was initialised.

Example

The following code:

class TopLevel extends Component {
  a := 42
  val a = UInt(8 bits)
}

will throw:

Exception in thread "main" java.lang.NullPointerException
  ***
  Source file location of the a := 42 assignement via the stack trace
  ***

A fix could be:

class TopLevel extends Component {
  val a = UInt(8 bits)
  a := 42
}
Issue explanation :
SpinalHDL is not a language, it is a Scala library, which means it obeys the same rules as the Scala general purpose programming language. When you run your SpinalHDL hardware description to generate the corresponding VHDL/Verilog RTL, your SpinalHDL hardware description will be executed as a Scala programm, and a will be a null reference until the program executes val a = UInt(8 bits), so trying to assign it before then will result in a NullPointerException.