how to reset drop down list(go to first option) when pushing any other button in App designer

I created dropDown list like following:
% Create ElementLisDropDown
            app.ElementLisDropDown = uidropdown(app.SelectionofTargetMaterialPanel);
            app.ElementLisDropDown.Items = {'Select an element', ' 1: HYDROGEN', ' 2: HELIUM', ' 3: LITHIUM', ' 4: BERYLLIUM', ' 5: BORON'} ;
            app.ElementLisDropDown.ItemsData = {'1', '2', '3', '4', '5'};
            app.ElementLisDropDown.ValueChangedFcn = createCallbackFcn(app, @ElementLisDropDownValueChanged, true);
            app.ElementLisDropDown.Position = [20 42 151 22];
            app.ElementLisDropDown.Value = '1';
            
In the callbacks for other button,  I added the following code: 
app.ElementLisDropDown.Value=app.ElementLisDropDown.Items(1);

but I'm getting this error:
Error using matlab.ui.control.internal.model.ExactlyOneSelectionStrategy/validateValuePresentInItemsData (line 236)
'Value' must be an element defined in the 'ItemsData' property.

Answer 1

[Yvan Lengwiler]

Your ItemsData are strings

app.ElementLisDropDown.ItemsData = {'1', '2', '3', '4', '5'};

but the value you assign is an integer

app.ElementLisDropDown.Value=app.ElementLisDropDown.Items(1);

Maybe if you said this instead?

app.ElementLisDropDown.Value=app.ElementLisDropDown.Items('1');