DevExpress XtraEditors LookupEdit does not show Popup

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C#

DevExpress

Win8-Desktop

I´m using Devexpress Xtragrid and want to add a LookupEdit

It does not show the Popup when preassing the down Arrow in the field
For teh Data I´m using a simple testclass

public class mx
{
    int mxval;
    string mxtext;

    public int MXval { get { return mxval; } set { mxval = value; } }
    public string MXtext { get { return mxtext; } set { mxtext = value; } }

    public mx(int i, string t)
    {
        mxval = i;
        mxtext = t;
    }
}
public List<mx> mxList = new List<mx>();
public List<mx> testmx
{
    get { return mxList; }
}

Which is filled with 3 Entries :

testmx.Add(new mx(0, ""));
testmx.Add(new mx(1, "Eins"));
testmx.Add(new mx(2, "Zwei"));

The DataSource is set to a BindingSource which uses this List
The DIsplay Member is set to MXText and Valuemember to MXValue

Also the Property-Editor fields of the Lookup-Edit were set by the GUI, so the Binding and Field names which wre selected by gui an not written in the text editor work correct
BUT the Pupo does nnot show , when running the App

Any ideas ??