v1.1537.0

Fix sidebar container initialization when "clean" theme is used

When the "clean" theme is used, the sidebar's toggle button is not created and
constructing the `DragHandler` failed because it was passed a `null` target.
Update the types in `ToolbarController.sidebarToggleButton` to reflect that the
value can be null and handle this in `Sidebar` by not initializing
`_dragResizeHandler` in this case.