Make RicHistogramPlotTools robust against null plots, data sources, and missing singletons

Guard all public entry points against null plot/data-source/curve/ensemble arguments, null-check RiaGuiApplication::instance() and RimMainPlotCollection::current() before dereferencing, and skip null curves/data sources when collecting existing data sources.

ApplicationLibCode/ProjectDataModel/Histogram/RimHistogramCurve.cpp

@@ -557,7 +557,7 @@ void RimHistogramCurve::loadAndUpdateDataAndPlot()
     plot->updateConnectedEditors();
 
     RiuPlotMainWindow* mainPlotWindow = RiaGuiApplication::instance()->mainPlotWindow();
-    mainPlotWindow->updateMultiPlotToolBar();
+    if ( mainPlotWindow ) mainPlotWindow->updateMultiPlotToolBar();
 
     dataChanged.send();
 }