handle empty sample (#188)

* handle empty sample

* code cleaning

src/easyreflectometry/sample/collections/sample.py

@@ -124,18 +124,20 @@ def _enable_changes_to_outermost_layers(self):
         Superphase can change thickness and roughness.
         Subphase can change thickness.
         """
-        self.superphase.thickness.enabled = True
-        self.superphase.roughness.enabled = True
-        self.subphase.thickness.enabled = True
+        if len(self) != 0:
+            self.superphase.thickness.enabled = True
+            self.superphase.roughness.enabled = True
+            self.subphase.thickness.enabled = True
 
     def _disable_changes_to_outermost_layers(self):
         """No allowed to change the outermost layers of the sample.
         Superphase can change thickness and roughness.
         Subphase can change thickness.
         """
-        self.superphase.thickness.enabled = False
-        self.superphase.roughness.enabled = False
-        self.subphase.thickness.enabled = False
+        if len(self) != 0:
+            self.superphase.thickness.enabled = False
+            self.superphase.roughness.enabled = False
+            self.subphase.thickness.enabled = False
 
     # Representation
     def as_dict(self, skip: Optional[List[str]] = None) -> dict:

tests/model/test_model.py

@@ -337,6 +337,17 @@ def test_remove_assembly_with_interface_refl1d(self):
     #     assert_equal(len(mod.interface()._wrapper.storage['item']), 1)
     #     assert_equal(len(mod.interface()._wrapper.storage['layer']), 2)
 
+    def test_remove_all_assemblies(self):
+        # when
+        mod = Model()
+
+        # Then
+        mod.remove_assembly(0)
+        mod.remove_assembly(0)
+
+        # Expect
+        assert_equal(len(mod.sample), 0)
+
     def test_resolution_function(self):
         mock_resolution_function = MagicMock()
         interface = CalculatorFactory()