Issue with accesing instance member from this scope is not yet implemented.

Try creating a new method, for example InitUI, and move the initialization of the NewImage variable there

custom_ui := class:

    NewTexture<protected> : texture = Widgets.Images.T_Robbery_MoneyBill_Bg

    var MainCanvas<private>:canvas = canvas{}
 
    InitUI():void=
          var NewImage<private>:texture_block = texture_block {DefaultImage := NewTexture, DefaultDesiredSize := vector2{X:=300.0, Y:=170.0}}