FEM: ResultMechanical add migration code f0r old result objects

Stats containing (min, avg, max) converted to (min, max)
Add Messagebox if histogram requested, but not available

src/Mod/Fem/femguiobjects/_ViewProviderFemResultMechanical.py

@@ -442,7 +442,13 @@ class _TaskPanelFemResultShow:
             self.none_selected(True)
 
     def show_histogram_clicked(self):
-        plt.show()
+        if len(plt.get_fignums()) > 0:
+            plt.show()
+        else:
+            QtGui.QMessageBox.information(None,
+                self.result_obj.Label + " - Information",
+                "No histogram available.\nPlease select a result type first."
+            )
 
     def user_defined_text(self, equation):
         FreeCAD.FEM_dialog["results_type"] = "user"

src/Mod/Fem/femobjects/_FemResultMechanical.py

@@ -286,7 +286,7 @@ class _FemResultMechanical():
 
         # initialize the Stats with the appropriate count of items
         # see fill_femresult_stats in femresult/resulttools.py
-        zero_list = 39 * [0]
+        zero_list = 26 * [0]
         obj.Stats = zero_list
 
     # standard Feature methods
@@ -315,6 +315,14 @@ class _FemResultMechanical():
             obj.vonMises = obj.StressValues
             obj.removeProperty("StressValues")
 
+        # migrate old result objects, because property "Stats"
+        # consisting of min, avg, max values was reduced to min, max in commit ???????
+        if len(obj.Stats) == 39:
+            temp = obj.Stats
+            for i in range(12, -1, -1):
+                del temp [3 * i + 1]
+            obj.Stats = temp
+
     def __getstate__(self):
         return self.Type