Initialize Access Type - uninitialized unconstraind allocation not allowed

Question

I'm currently struggeling a bit when trying to initialize an array access. I've got the following in my generic package specification:

generic
    type Item_Type is private;
    with function "+"(Left: Item_Type; Right: Item_Type) return Item_Type;
package Parallel_Algorithms is

    type Array_Type is array(Natural range <>) of Item_Type;
    type Array_Access_Type is access all Array_Type;
    ...
end Parallel_Algorithms;

where Item_Type should be the type of my generic package. I would now like to instantiate the Array_Access_Type in another procedure since I need to pass it to a function also specified in the package. The package is instantiated in the following way:

package Natural_Parallel is new Parallel_Algorithms(Item_Type => Natural,
                                                    "+"       => "+");

I found some forum thread where the answer was basically like that:

Arr_Acc: Array_Access_Type := new Array_Type;

However I get the following error I don't quite understand:

uninitialized unconstrained allocation not allowed
qualified expression or constraint with array bounds required

What is the correct way to instantiate the access type with some values inside of the array for later use?