Don't crash when setting JS theme value to null (#16210)

Closes #16035

In v3 it was possible to unset a specific color namespace by setting
doing something like this:

```js
export default {
  theme: {
    extend: {
      colors: {
        red: null,
      },
    },
  },
}
```

This pattern would crash in v4 right now due to the theme access
function not being able to work on the red property being a `null`. This
PR fixes this crash.

However it leaves the behavior as-is for now so that the red namespace
_defined via CSS will still be accessible_. This is technically
different from v3 but fixing this would be more work as we only allow
unsetting top-level namespaces in the interop layer (via the
non-`extend`-theme-object). I would recommend migrating to the v4 API
for doing these partial namespace resets if you want to get rid of the
defaults in v4:

```css
@theme {
  --color-red-*: initial;
}
```

## Test plan

The crash was mainly captured via the test in `compat/config.test.ts`
but I've added two more tests across the different levels of
abstractions so that it's clear what `null` should be doing.

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Fix a crash when setting JS theme values to `null` ([#16210](https://github.com/tailwindlabs/tailwindcss/pull/16210))
 - Ensure CSS variables in arbitrary values are properly decoded ([#16206](https://github.com/tailwindlabs/tailwindcss/pull/16206))
 - Ensure that the `containers` JS theme key is added to the `--container-*` namespace ([#16169](https://github.com/tailwindlabs/tailwindcss/pull/16169))
 - Fix missing `@keyframes` definition ([#16237](https://github.com/tailwindlabs/tailwindcss/pull/16237))

packages/tailwindcss/src/compat/apply-config-to-theme.test.ts

@@ -223,3 +223,39 @@ test('converts opacity modifiers from decimal to percentage values', () => {
   expect(theme.resolve('20', ['--opacity'])).toEqual('20%')
   expect(theme.resolve('25', ['--opacity'])).toEqual('25%')
 })
+
+test('handles setting theme keys to null', async () => {
+  let theme = new Theme()
+  let design = buildDesignSystem(theme)
+
+  theme.add('--color-blue-400', 'blue', ThemeOptions.DEFAULT)
+  theme.add('--color-blue-500', '#3b82f6')
+  theme.add('--color-red-400', 'red', ThemeOptions.DEFAULT)
+  theme.add('--color-red-500', '#ef4444')
+
+  let { resolvedConfig, replacedThemeKeys } = resolveConfig(design, [
+    {
+      config: {
+        theme: {
+          extend: {
+            colors: {
+              blue: null,
+            },
+          },
+        },
+      },
+      base: '/root',
+      reference: false,
+    },
+  ])
+  applyConfigToTheme(design, resolvedConfig, replacedThemeKeys)
+
+  expect(theme.namespace('--color')).toMatchInlineSnapshot(`
+    Map {
+      "blue-400" => "blue",
+      "blue-500" => "#3b82f6",
+      "red-400" => "red",
+      "red-500" => "#ef4444",
+    }
+  `)
+})

packages/tailwindcss/src/compat/config.test.ts

@@ -1628,3 +1628,54 @@ test('old theme values are merged with their renamed counterparts in the CSS the
 
   expect(didCallPluginFn).toHaveBeenCalled()
 })
+
+test('handles setting theme keys to null', async () => {
+  let compiler = await compile(
+    css`
+      @theme default {
+        --color-red-50: oklch(0.971 0.013 17.38);
+        --color-red-100: oklch(0.936 0.032 17.717);
+      }
+      @config "./my-config.js";
+      @tailwind utilities;
+      @theme {
+        --color-red-100: oklch(0.936 0.032 17.717);
+        --color-red-200: oklch(0.885 0.062 18.334);
+      }
+    `,
+    {
+      loadModule: async () => {
+        return {
+          module: {
+            theme: {
+              extend: {
+                colors: {
+                  red: null,
+                },
+              },
+            },
+          },
+          base: '/root',
+        }
+      },
+    },
+  )
+
+  expect(compiler.build(['bg-red-50', 'bg-red-100', 'bg-red-200'])).toMatchInlineSnapshot(`
+    ":root, :host {
+      --color-red-50: oklch(0.971 0.013 17.38);
+      --color-red-100: oklch(0.936 0.032 17.717);
+      --color-red-200: oklch(0.885 0.062 18.334);
+    }
+    .bg-red-50 {
+      background-color: var(--color-red-50);
+    }
+    .bg-red-100 {
+      background-color: var(--color-red-100);
+    }
+    .bg-red-200 {
+      background-color: var(--color-red-200);
+    }
+    "
+  `)
+})

packages/tailwindcss/src/compat/config/resolve-config.test.ts

@@ -254,3 +254,48 @@ test('theme keys can read from the CSS theme', () => {
     new Set(['colors', 'accentColor', 'placeholderColor', 'caretColor', 'transitionColor']),
   )
 })
+
+test('handles null as theme values', () => {
+  let theme = new Theme()
+  theme.add('--color-red-50', 'red')
+  theme.add('--color-red-100', 'red')
+
+  let design = buildDesignSystem(theme)
+
+  let { resolvedConfig, replacedThemeKeys } = resolveConfig(design, [
+    {
+      config: {
+        theme: {
+          colors: ({ theme }) => ({
+            // Reads from the --color-* namespace
+            ...theme('color'),
+          }),
+        },
+      },
+      base: '/root',
+      reference: false,
+    },
+    {
+      config: {
+        theme: {
+          extend: {
+            colors: {
+              red: null,
+            },
+          },
+        },
+      },
+      base: '/root',
+      reference: false,
+    },
+  ])
+
+  expect(resolvedConfig).toMatchObject({
+    theme: {
+      colors: {
+        red: null,
+      },
+    },
+  })
+  expect(replacedThemeKeys).toEqual(new Set(['colors']))
+})

packages/tailwindcss/src/compat/plugin-functions.ts

@@ -224,7 +224,7 @@ function get(obj: any, path: string[]) {
     let key = path[i]
 
     // The key does not exist so concatenate it with the next key
-    if (obj[key] === undefined) {
+    if (obj?.[key] === undefined) {
       if (path[i + 1] === undefined) {
         return undefined
       }