#14450 Guard against missing data source in RFT plot

RimWellRftPlot::dataSource() returned a default constructed variant when no source was selected. The variant then held a null RimSummaryCase pointer, and RicCreateRftPlotsFeature dereferenced it. Add std::monostate as first alternative to represent no data source.

ApplicationLibCode/ProjectDataModel/Flow/RimWellRftPlot.cpp

@@ -1897,7 +1897,7 @@ void RimWellRftPlot::initializeDataSources( RimWellRftPlot* source )
 //--------------------------------------------------------------------------------------------------
 ///
 //--------------------------------------------------------------------------------------------------
-std::variant<RimSummaryCase*, RimSummaryEnsemble*> RimWellRftPlot::dataSource() const
+std::variant<std::monostate, RimSummaryCase*, RimSummaryEnsemble*> RimWellRftPlot::dataSource() const
 {
     // Return the first selected ensemble, if any
     // If no ensemble is selected, return the first selected summary case, if any

ApplicationLibCode/ProjectDataModel/Flow/RimWellRftPlot.h

@@ -88,7 +88,8 @@ public:
 
     int branchIndex() const;
 
-    std::variant<RimSummaryCase*, RimSummaryEnsemble*> dataSource() const;
+    // The monostate is used to indicate that no data source is selected
+    std::variant<std::monostate, RimSummaryCase*, RimSummaryEnsemble*> dataSource() const;
 
     static const char* plotNameFormatString();