为什么这个声明的组合框会抛出一个调用异常？

Question

我在xaml的主窗口中设置了一个组合框，但是当我尝试获取索引时，我在代码中获得了一个空引用。为什么呢？

ComboBox Name="cmbo_addvar_datatype" Margin="4,0,1,0" Grid.Row="10" Grid.ColumnSpan="2"  ToolTip="Select a datatype for the variable being 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 with a 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 As String = cmbo_addvar_datatype.SelectedIndex