lttng: Fix ust memory analysis help text display

lttng: Fix ust memory analysis help text display

When the analysis cannot be executed due to missing requirements, the
user can request the help text to get the reason why. However, this
was not working and caused an exception because the requirements are
only instantiated when the trace is open.

Change-Id: I43ca26b964467711bea10bf7ed52eba2988e3fd9
Signed-off-by: Bernd Hufmann <Bernd.Hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/68236
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>