vb.net - Why is this declared combobox throwing an invocation exception? -

vb.net - Why is this declared combobox throwing an invocation exception? -

i've got combobox set in main window in xaml, when seek index null reference in code. why?

combobox name="cmbo_addvar_datatype" margin="4,0,1,0" grid.row="10" grid.columnspan="2" tooltip="select datatype variable beingness added." selectionchanged="test_addedvar"> <comboboxitem isselected="true" tooltip="true/flase value.">boolean</comboboxitem> <comboboxitem tooltip="">little</comboboxitem> <comboboxitem>big</comboboxitem> <comboboxitem tooltip="text value">string</comboboxitem> <comboboxitem tooltip="value decimal point">decimal</comboboxitem> <comboboxitem tooltip="list of strings.">string list</comboboxitem> <comboboxitem tooltip="array of type little">array(little)</comboboxitem> <comboboxitem tooltip="array of type big">array(big)</comboboxitem> </combobox>

mainwindow.xaml.vb

dim datatype string = cmbo_addvar_datatype.selectedindex

vb.net xaml