If the RadSpreadsheet is declared as a child content in the RadBusyIndicator, there isn't any tab selected as active in the SheetSelector.

Workaround: Declare the busy indicator after the RadSpreadsheet not as its child.

For example, instead of:

<telerik:RadBusyIndicator>
    <telerik:RadSpreadsheet x:Name="radSpreadsheet" />
</telerik:RadBusyIndicator>

Do:

<Grid>
    <telerik:RadSpreadsheet x:Name="radSpreadsheet" />
    <telerik:RadBusyIndicator />
</Grid>