How to get selecetedindex value from radcombobox

<telerik:RadComboBox ItemsSource="{Binding Path=Sectors}" SelectedIndex="{Binding Path=SelectedPath,Mode=TwoWay}" SelectedValue="{Binding Path=SelectedSector,Mode=TwoWay}" IsEnabled="{Binding Path=IsPathEnabled}" Width="120" Height="25" VerticalAlignment="Center" Margin="5,0,2,0" x:Name="cbxPaths">
                <i:Interaction.Triggers>
                    <i:EventTrigger EventName="DropDownOpened">
                        <commands:EventToCommand Command="{Binding Path=DropDownOpenedCommand}" ></commands:EventToCommand>
                    </i:EventTrigger>
                </i:Interaction.Triggers>
   </telerik:RadComboBox>

How to get selecetedindex value from combobox

private int selectedPath = -1;
public int SelectedPath
        {
            get
            {
                return selectedPath;
            }
            set
            {
                selectedPath = value;
                RaisePropertyChanged("SelectedPath");
            }
        }

if(selectedpath == cbxPaths.SelectedIndex)
{
    //some code Here
}

If i run the program i got null exception value(cbxPaths.SelectedIndex).

Object reference not set to an instance of an object.

Any Idea?