[FIX] web: ActionManager: protect set/get scrolltop methods

Here is what could happen before this commit: open a record (let's say
project.task) in form view.  Then, on the sidebar (actions), choose
Merge Selected Tasks), choose a task, and click on Merge. What happens
next is a traceback.

Here is a more precise description of what happened:
- the formview dialog for the merge action closes, which triggers a
  reload for the main formview
- the merge action returned an action description for an act_window
- the action manager instantiated a new viewmanager and prepare the form
  view, which is an async operation (need to load some data to display
  the form)
- the previous view manager (initial one) completes its reload, which
  makes it rerender the view, which triggers a DOM_updated event
- the new view manager see this event, and tries to call getScrollTop on
  the current view controller, but there are no controller yet, it is
  not ready.

We could argue that some of these events should not happen (for example,
if we will open a new view manager action, why do we reload the current
view manager?).  But solving these issues is a more deep architectural
problem (we probably will rewrite the actionmanager/view manager layer
at some point).

It could also be argued that in any case, the method getScrollTop should
not crash (same for setScrollTop).  Sadly, these methods are fragile,
because some changes in the loading behaviour of a view may break them.
So, in any case, protecting them against unexpected traceback is a
worthwile investment.

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