Force a child class to initialize a variable

I have a class Foo that has a field _customObject that must be initialized. I also have a class Bar that inherits from Foo:

public abstract class Foo
{
    protected CustomObject _customObject;

    public Foo()
    {
        // Do stuff
    }
    // Other methods that use _customObject
}

public class Bar : Foo
{
    // Constructor and other methods
}

I can not initialize the object _customObject in Foo because every child inherited contains a different child of CustomObject, so it must be initialized in every child class:

public class Bar : Foo
{
    public Bar()
    {
        _customObject = new CustomObjectInherited1();
    }
}


public class Baz : Foo
{
    public Baz()
    {
        _customObject = new CustomObjectInherited2();
    }
}

Other people are going to implement new classes that inherit from Foo, so I was wondering if there is a way that an error in build time is shown, similar to when an abstract method is not implemented. If CustomObject is not initialized, a NullReferenceException will be thrown due to the use of the _customObject variable, ending in an application crash.