Move the _is_created assignment to the top (napari#5078)

# Description

The `_is_creating` flag is set too late in the drawing process.

If one clicks two times while `polygon` or `path` is selected, an error
is thrown. I also added a test and a fix for this scenario.

## Type of change

- [X] Bug-fix (non-breaking change which fixes an issue)
- [X] New feature (non-breaking change which adds functionality)

---------

Co-authored-by: Markus Stabrin <stabrin@MonsterMac.local>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Juan Nunez-Iglesias <jni@fastmail.com>
Co-authored-by: Peter Sobolewski <76622105+psobolewskiPhD@users.noreply.github.com>
Co-authored-by: Genevieve Buckley <30920819+GenevieveBuckley@users.noreply.github.com>
Co-authored-by: Andy Sweet <andrew.d.sweet@gmail.com>
Co-authored-by: Grzegorz Bokota <bokota+github@gmail.com>
Co-authored-by: Wouter-Michiel Vierdag <michiel.vierdag@embl.de>
Co-authored-by: Wouter-Michiel Vierdag <w-mv@hotmail.com>

napari/layers/shapes/_shapes_mouse_bindings.py

@@ -311,12 +311,12 @@ def initiate_polygon_draw(
     coordinates : Tuple[float, ...]
         A tuple with the coordinates of the initial vertex in image data space.
     """
+    layer._is_creating = True
     data = np.array([coordinates, coordinates])
     layer.add(data, shape_type='path', gui=True)
     layer.selected_data = {layer.nshapes - 1}
     layer._value = (layer.nshapes - 1, 1)
     layer._moving_value = copy(layer._value)
-    layer._is_creating = True
     layer._set_highlight()
 
 
@@ -383,6 +383,10 @@ def add_vertex_to_path(
     vertices = layer._data_view.shapes[index].data
     vertices = np.concatenate((vertices, [coordinates]), axis=0)
     value = layer.get_value(event.position, world=True)
+    # If there was no move event between two clicks value[1] is None
+    # and needs to be taken care of.
+    if value[1] is None:
+        value = layer._moving_value
     layer._value = (value[0], value[1] + 1)
     layer._moving_value = copy(layer._value)
     layer._data_view.edit(index, vertices, new_type=new_type)

napari/layers/shapes/_tests/test_shapes_mouse_bindings.py

@@ -702,6 +702,75 @@ def test_after_in_add_mode_shape(mode, create_known_shapes_layer):
     assert len(layer.selected_data) == 0
 
 
+@pytest.mark.parametrize(
+    'mode',
+    [
+        'add_polygon',
+        'add_path',
+    ],
+)
+def test_clicking_the_same_point_is_not_crashing(
+    mode, create_known_shapes_layer
+):
+    layer, n_shapes, _ = create_known_shapes_layer
+
+    layer.mode = mode
+    position = tuple(layer.data[0][0])
+
+    for _ in range(2):
+        event = read_only_mouse_event(type='mouse_press', position=position)
+        mouse_press_callbacks(layer, event)
+
+        event = read_only_mouse_event(type='mouse_release', position=position)
+        mouse_release_callbacks(layer, event)
+
+    # If there was no move event between the two clicks, we expect value[1] must be None
+    assert layer.get_value(event.position, world=True)[1] is None
+
+
+@pytest.mark.parametrize(
+    'mode',
+    [
+        'add_polygon',
+        'add_path',
+    ],
+)
+def test_is_creating_is_false_on_creation(mode, create_known_shapes_layer):
+    layer, n_shapes, _ = create_known_shapes_layer
+
+    layer.mode = mode
+    position = tuple(layer.data[0][0])
+
+    def is_creating_is_True(event):
+        assert event.source._is_creating
+
+    def is_creating_is_False(event):
+        assert not event.source._is_creating
+
+    assert not layer._is_creating
+    layer.events.set_data.connect(is_creating_is_True)
+
+    event = read_only_mouse_event(type='mouse_press', position=position)
+    mouse_press_callbacks(layer, event)
+
+    assert layer._is_creating
+
+    event = read_only_mouse_event(type='mouse_release', position=position)
+    mouse_release_callbacks(layer, event)
+
+    assert layer._is_creating
+
+    layer.events.set_data.disconnect(is_creating_is_True)
+    layer.events.set_data.connect(is_creating_is_False)
+    end_click = read_only_mouse_event(
+        type='mouse_double_click',
+        position=position,
+    )
+    mouse_double_click_callbacks(layer, end_click)
+
+    assert not layer._is_creating
+
+
 @pytest.mark.parametrize('mode', ['select', 'direct'])
 def test_unselect_select_shape(mode, create_known_shapes_layer):
     """Select a shape by clicking on one in select mode."""