Items collection must be empty before using ItemsSource.

This error occurs when you try to add Children to a control which already has a item source as a result there is a lot of confusion to the parser regarding which one to choose.

Any children you add to the control are a part of its item collection consider for example

<listview margin="8,9,11,125" name="listView1" itemssource="{Binding}" minwidth="250" minheight="200" columnspan="2" rowspan="2">
<listview.view>
<gridview>
<gridviewcolumn header="ID" displaymemberbinding="{Binding Path=ID}"></gridviewcolumn>
<gridviewcolumn header="Company" displaymemberbinding="{Binding Path=Company}"></gridviewcolumn>
<gridviewcolumn header="Email" displaymemberbinding="{Binding Path=Email}"></gridviewcolumn>
<gridviewcolumn header="Website" displaymemberbinding="{Binding Path=Website}"></gridviewcolumn>
<gridviewcolumn header="Phone" displaymemberbinding="{Binding Path=Phone}"></gridviewcolumn>
<gridviewcolumn header="Address" displaymemberbinding="{Binding Path=Address}"></gridviewcolumn>
<gridviewcolumn header="Field of Development" displaymemberbinding="{Binding Path=Field}"></gridviewcolumn>
<gridviewcolumn header="Notes" displaymemberbinding="{Binding Path=Notes}"></gridviewcolumn>
</gridview>
</listview.view>
<Button Margin="0,0,-326.634,-49.995" Name="buttonReset" HorizontalAlignment="Right" Grid.Row="6" Width="99.99" Height="31.123" VerticalAlignment="Bottom" Click="buttonReset_Click">Reset</Button>
</listview>

Now here I have added a button in the last as a part of Item collection for the list view so it gives me that error

When I remove that code everything works fine.We cannot insert items and set items source at same time.