Ensure list access is performed after the length check (#2265)

Author: Qu4tro

poetry/masonry/utils/package_include.py

@@ -34,13 +34,12 @@ def refresh(self):  # type: () -> PackageInclude
         return self.check_elements()
 
     def check_elements(self):  # type: () -> PackageInclude
-        root = self._elements[0]
-
         if not self._elements:
             raise ValueError(
                 "{} does not contain any element".format(self._base / self._include)
             )
 
+        root = self._elements[0]
         if len(self._elements) > 1:
             # Probably glob
             self._is_package = True

tests/masonry/utils/test_package_include.py

@@ -41,3 +41,12 @@ def test_package_include_with_no_python_files_in_dir():
         PackageInclude(base=with_includes, include="not_a_python_pkg")
 
     assert str(e.value) == "not_a_python_pkg is not a package."
+
+
+def test_package_include_with_non_existent_directory():
+    with pytest.raises(ValueError) as e:
+        PackageInclude(base=with_includes, include="not_a_dir")
+
+    err_str = str(with_includes / "not_a_dir") + " does not contain any element"
+
+    assert str(e.value) == err_str