#13872 WellLog: Fix crash when deleting ensemble well log curve sets

Detach m_qwtPlotCurveForLegendText before the viewer check so it is
always detached before deletion. Add null check for curves in
detachPlotCurves, and explicitly call detachPlotCurves from
RimWellLogTrack::detachAllPlotItems to ensure curves are detached
before the track is destroyed.

ApplicationLibCode/ProjectDataModel/WellLog/WellLogTrack/RimWellLogTrack.cpp

@@ -378,6 +378,10 @@ void RimWellLogTrack::detachAllPlotItems()
     {
         plotObjects->detachFromQwt();
     }
+    if ( m_ensembleWellLogCurveSet )
+    {
+        m_ensembleWellLogCurveSet->detachPlotCurves();
+    }
 }
 
 //--------------------------------------------------------------------------------------------------