Write test cases for a partial class that extends another class in C#

I have a C# code as follows

public partial class ChildClass : ParentClass
{   
...
...
    public ChildClass() : base(SomeCore.Context.Item.Fields["Parameters Template Name"].ConvertToString())
    {
    this.memVarA = //reading from some Context
        this.memVarA = //reading from some Context    
    }
}

I want to create a test cases for ChildClass. In the test class I have this code:

[TestMethod()]
public void ItemsListShouldHaveOnly4Items()
{
    ChildClass ChildClass = new ChildClass();
    PrivateObject privateObject = new PrivateObject(ChildClass);
    privateObject.Invoke("aPrivateMethod");
    var retVal = privateObject.Invoke("getItemsList");
    Assert.AreEqual(4, retVal);
}

The test breaks with exception : System.NullReferenceException: Object reference not set to an instance of an Object.

How can I go about writing test cases for this class ?