fix: prevent deep_set crash on nullish nested values

.changeset/deep-set-nullish.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: prevent `deep_set` crash on nullish nested values

packages/kit/src/runtime/form-utils.js

@@ -466,8 +466,8 @@ export function deep_set(object, keys, value) {
 		check_prototype_pollution(key);
 
 		const is_array = /^\d+$/.test(keys[i + 1]);
-		const exists = Object.hasOwn(current, key);
-		const inner = current[key];
+		const inner = Object.hasOwn(current, key) ? current[key] : undefined;
+		const exists = inner != null;
 
 		if (exists && is_array !== Array.isArray(inner)) {
 			throw new Error(`Invalid array key ${keys[i + 1]}`);

packages/kit/src/runtime/form-utils.spec.js

@@ -425,4 +425,20 @@ describe('deep_set', () => {
 		// @ts-ignore
 		expect(Object.prototype.toString.property).toBeUndefined();
 	});
+
+	test('creates nested object when intermediate value is undefined', () => {
+		const target = { nested: undefined };
+
+		deep_set(target, ['nested', 'name'], 'hello');
+
+		expect(target).toEqual({ nested: { name: 'hello' } });
+	});
+
+	test('creates nested object when intermediate value is null', () => {
+		const target = { nested: null };
+
+		deep_set(target, ['nested', 'name'], 'hello');
+
+		expect(target).toEqual({ nested: { name: 'hello' } });
+	});
 });