Fix crashes reported by crash telemetry

* Recompute well cell arrays when the grids change

computeWellCellsPrGrid() returned early whenever the arrays had been computed once, so the
size check below it was unreachable and a stale array was kept when the grids changed. The
too-short array triggered an assert in RivReservoirViewPartMgr::computeNativeVisibility.

Return early only when the cached arrays still match the current set of grids.

* Guard against missing GUI application when applying style sheet

RiaGuiApplication::instance() does a dynamic_cast and returns null outside a GUI context. The
assert guarding this is compiled out in release, so the null pointer was dereferenced.

* Guard against missing case in flow characteristics plot field change

The case field can be set to nothing from the UI. RimEclipseResultCase::defaultFlowDiagSolution()
and reservoirViews() were then called on a null pointer.

* Guard against missing case and out of range time steps when updating flow characteristics plot

onLoadDataAndUpdate() dereferenced the case without checking, although the same function handles
a missing case further down. The time step indices come from the flow diagnostics solution, and
were used to index arrays sized by the number of case time steps without a range check.

ApplicationLibCode/UserInterface/RiuGuiTheme.cpp

@@ -381,10 +381,13 @@ bool RiuGuiTheme::applyStyleSheet( RiaDefines::ThemeEnum theme )
     {
         if ( styleSheetFile.open( QIODevice::ReadOnly ) )
         {
-            RiaGuiApplication* app        = RiaGuiApplication::instance();
-            QString            styleSheet = styleSheetFile.readAll();
+            QString styleSheet = styleSheetFile.readAll();
             preparseStyleSheet( theme, styleSheet );
-            app->setStyleSheet( styleSheet );
+
+            if ( RiaGuiApplication* app = RiaGuiApplication::instance() )
+            {
+                app->setStyleSheet( styleSheet );
+            }
         }
         return true;
     }