[FEM] fix subsequent histogram plot call

fixes #6975

src/Mod/Fem/femtaskpanels/task_result_mechanical.py

@@ -382,11 +382,19 @@ class _TaskPanel:
         if len(plt.get_fignums()) > 0:
             plt.show()
         else:
-            QtGui.QMessageBox.information(
-                None,
-                self.result_obj.Label + " - " + translate("FEM","Information"),
-                translate("FEM","No histogram available.\nPlease select a result type first.")
-            )
+            # if the plot was closed and subsequently the Histogram button was pressed again
+            # we have valid settings, but must restore the dialog to refill the plot content
+            # see https://github.com/FreeCAD/FreeCAD/issues/6975
+            if FreeCAD.FEM_dialog["results_type"] != "None":
+                self.restore_result_dialog()
+            if len(plt.get_fignums()) > 0:
+                plt.show()
+            else:
+                QtGui.QMessageBox.information(
+                    None,
+                    self.result_obj.Label + " - " + translate("FEM","Information"),
+                    translate("FEM","No histogram available.\nPlease select a result type first.")
+                )
 
     def user_defined_text(self, equation):
         FreeCAD.FEM_dialog["results_type"] = "user"