Fix #161987: Avoid compositor modifier UI crash with one input

The Sequencer Compositor modifier UI checked whether the node group had
at least one interface input before accessing the second input used for
the optional mask.

A node group with exactly one input therefore caused an out-of-bounds
access when an ID mask was assigned and the Strip Modifiers panel was
redrawn.

Require at least two interface inputs before validating the mask input.

Pull Request: https://projects.blender.org/blender/blender/pulls/162031

tests/python/ui_simulate/test_sequencer.py

@@ -0,0 +1,60 @@
+# SPDX-FileCopyrightText: 2026 Blender Authors
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+"""
+This file does not run anything, its methods are accessed for tests by ``run_blender_setup.py``.
+"""
+import os
+
+import modules.ui_test_utils as ui
+
+
+def compositor_modifier_mask_with_single_input():
+    import bpy
+
+    _, t, window = ui.test_window()
+
+    scene = window.scene
+    window.workspace.sequencer_scene = scene
+    sequence_editor = scene.sequence_editor_create()
+    movie_path = os.path.normpath(
+        os.path.join(os.path.dirname(__file__), "..", "..", "files", "compositor", "motion.mp4")
+    )
+    strip = sequence_editor.strips.new_movie(
+        name="Movie",
+        filepath=movie_path,
+        channel=1,
+        frame_start=1,
+    )
+    strip.select = True
+    sequence_editor.active_strip = strip
+
+    node_group = bpy.data.node_groups.new("Compositor Nodes", 'CompositorNodeTree')
+    node_group.interface.new_socket(name="Image", in_out='INPUT', socket_type='NodeSocketColor')
+    node_group.interface.new_socket(name="Image", in_out='OUTPUT', socket_type='NodeSocketColor')
+    group_input = node_group.nodes.new('NodeGroupInput')
+    group_output = node_group.nodes.new('NodeGroupOutput')
+    node_group.links.new(group_input.outputs["Image"], group_output.inputs["Image"])
+    interface_items = node_group.interface.items_tree
+    t.assertEqual(sum(item.in_out == 'INPUT' for item in interface_items), 1)
+    t.assertEqual(sum(item.in_out == 'OUTPUT' for item in interface_items), 1)
+
+    modifier = strip.modifiers.new("Compositor", 'COMPOSITOR')
+    modifier.node_group = node_group
+    modifier.show_expanded = True
+    mask = bpy.data.masks.new("Mask")
+
+    properties_area = ui.get_window_area_by_type(window, 'PROPERTIES')
+    yield
+    properties_area.spaces.active.context = 'STRIP_MODIFIER'
+    yield
+    t.assertEqual(properties_area.spaces.active.context, 'STRIP_MODIFIER')
+
+    modifier.input_mask_type = 'ID'
+    modifier.input_mask_id = mask
+    properties_area.tag_redraw()
+    yield
+
+    t.assertEqual(modifier.input_mask_type, 'ID')
+    t.assertIs(modifier.input_mask_id, mask)