NullReferenceException: Object reference not set to an instance of an object

And again:

I get this error (see title)

in this line of code:

 if(inventory.fieldContents[i].id != 0 && inventory.fieldContents[i].id <= manager.Items.Length){

All you need to know is the inventory class :

 class inv{
     var fieldContents :field[];
     var quickContents :field[];
     var lines :int = 5;
     var perLine :int = 10;
     var size :Vector2;
     var spacing :Vector2;
 }
 var inventory :inv;

and the field class:

 class field{
     public var id :int;
     public var amount:int;
 }

In the start function i also have this for being sure that there are enaugh fields:

 inventory.fieldContents = new field[inventory.lines * inventory.perLine + inventory.perLine];
 inventory.quickContents = new field[inventory.perLine];
 manager.hovered = new boolean[inventory.fieldContents.Length];

Although everything works as it shoulds this message is very ennoying. Can somebody help me solving the problem?

(BTW: The Cause is defenetly the fieldContents variable not the manager variable)