#14011 FlowRateCurve: Guard against null plot curve in updateCurveAppearance

When the user toggles "Show Curve" off, updateCurvePresentation runs
updateCurveVisibility, which detaches and deletes m_plotCurve, before
calling updateCurveAppearance. The override in RimWellFlowRateCurve
dereferenced m_plotCurve unconditionally and crashed. Match the null
guard already present in RimPlotCurve::updateCurveAppearance.

Fixes #14011.

ApplicationLibCode/ProjectDataModel/Flow/RimWellFlowRateCurve.cpp

@@ -183,6 +183,8 @@ void RimWellFlowRateCurve::updateCurveAppearance()
 {
     RimWellLogCurve::updateCurveAppearance();
 
+    if ( !m_plotCurve ) return;
+
     bool isLastCurveInGroup = false;
     {
         auto                                         wellLogTrack       = firstAncestorOrThisOfTypeAsserted<RimWellLogTrack>();