fix: allow nullable service registrations

src/di-container.ts

@@ -92,13 +92,12 @@ export class DIContainer implements IDIContainer {
 		if (options == null) {
 			throw new ReferenceError(`1 argument required, but only 0 present. ${DI_COMPILER_ERROR_HINT}`);
 		}
-		const instance = this.#constructInstance<T>(options);
 
-		if (instance == null) {
+		if (!this.has(options)) {
 			throw new InstantiationError(`The service wasn't found in the registry.`, {identifier: options.identifier});
 		}
 
-		return instance;
+		return this.#constructInstance<T>(options)!;
 	}
 
 	/**

test/di-container.test.ts

@@ -174,3 +174,16 @@ test(`Can construct services with class constructors. #2`, () => {
 	assert.deepEqual(cInstance.serviceB, bInstance);
 	assert.deepEqual(bInstance.serviceA, aInstance);
 });
+
+test(`Services that allow for nullable values can be constructed without exceptions. #1`, () => {
+	interface IServiceA {
+		foo: string;
+	}
+
+	type NullableServiceA = IServiceA | undefined;
+
+	const container = new DIContainer();
+
+	container.registerSingleton<NullableServiceA>(() => undefined, {identifier: "NullableServiceA"});
+	assert.doesNotThrow(() => container.get<NullableServiceA>({identifier: "NullableServiceA"}));
+});