Detailed error and warnings upon next() call (vercel#13539)

**First, apologies for a second PR on the same issue but I was working on this already so I thought I'd push it and let you decide which you want to merge.**

The PR is related to [13466](vercel#13466).

Based on my research, the error happens if the options are empty, null, or undefined. That's why I have decided that the most proper check would be using the! post-fix expression operator may assert that its operand is non-null and non-undefined. ``if (options == null)``

(Optional)
I have also added a warning, which warns the user if the passed "dev" variable is not a boolean.

It's my first PR on the "packages" part of the repo so I'd be glad to receive all kinds of critics. If you want me to change or remove anything, I'm open to suggestions.

---

Fixes vercel#13466

errors/invalid-server-options.md

@@ -0,0 +1,25 @@
+# It looks like the next instance is being instantiated incorrectly.
+
+#### Why This Error Occurred
+
+You have passed a null or undefined parameter to the next() call.
+
+#### Possible Ways to Fix It
+
+Make sure you are passing the variables properly:
+
+```js
+const app = next()
+```
+
+And make sure you're passing dev as shown in the examples below:
+
+```js
+const app = next({ dev: boolean })
+```
+
+### Useful Links
+
+- [custom-server-express](https://github.com/vercel/next.js/blob/6ca00bfe312c8d3fc5c20d25a9cd8d2741a29332/examples/custom-server-express/server.js#L6)
+- [custom-server](https://github.com/vercel/next.js/blob/6ca00bfe312c8d3fc5c20d25a9cd8d2741a29332/examples/custom-server/server.js#L6)
+- [custom-server-typescript](https://github.com/vercel/next.js/blob/6ca00bfe312c8d3fc5c20d25a9cd8d2741a29332/examples/custom-server-typescript/server/index.ts#L7)

packages/next/server/next.ts

@@ -13,6 +13,12 @@ type NextServerConstructor = Omit<ServerConstructor, 'staticMarkup'> & {
 function createServer(options: NextServerConstructor): Server {
   const standardEnv = ['production', 'development', 'test']
 
+  if (options == null) {
+    throw new Error(
+      'The server has not been instantiated properly. https://err.sh/next.js/invalid-server-options'
+    )
+  }
+
   if (
     !(options as any).isNextDevCommand &&
     process.env.NODE_ENV &&
@@ -22,6 +28,12 @@ function createServer(options: NextServerConstructor): Server {
   }
 
   if (options.dev) {
+    if (typeof options.dev !== 'boolean') {
+      console.warn(
+        "Warning: 'dev' is not a boolean which could introduce unexpected behavior. https://err.sh/next.js/invalid-server-options"
+      )
+    }
+
     const DevServer = require('./next-dev-server').default
     return new DevServer(options)
   }

test/integration/invalid-server-options/pages/index.js

@@ -0,0 +1 @@
+export default () => 'test'

test/integration/invalid-server-options/test/index.test.js

@@ -0,0 +1,65 @@
+import next from 'next'
+import { join } from 'path'
+const dir = join(__dirname, '../')
+const warningMessage =
+  "Warning: 'dev' is not a boolean which could introduce unexpected behavior. https://err.sh/next.js/invalid-server-options"
+
+describe('Invalid server options', () => {
+  test('next() called with no parameters should throw error', () => {
+    expect(() => next()).toThrowError(
+      'The server has not been instantiated properly. https://err.sh/next.js/invalid-server-options'
+    )
+  })
+
+  test('next() called with undefined parameter should throw error', () => {
+    expect(() => next(undefined)).toThrowError(
+      'The server has not been instantiated properly. https://err.sh/next.js/invalid-server-options'
+    )
+  })
+
+  test('next() called with null parameter should throw error', () => {
+    expect(() => next(null)).toThrowError(
+      'The server has not been instantiated properly. https://err.sh/next.js/invalid-server-options'
+    )
+  })
+
+  test('next() called with dev as string should send warning', () => {
+    const consoleSpy = jest.spyOn(console, 'warn')
+    const dev = 'string'
+    next({ dev, dir })
+
+    expect(consoleSpy).toHaveBeenCalledWith(warningMessage)
+  })
+
+  test('next() called with dev as number should send warning', () => {
+    const consoleSpy = jest.spyOn(console, 'warn')
+    const dev = 123
+    next({ dev, dir })
+
+    expect(consoleSpy).toHaveBeenCalledWith(warningMessage)
+  })
+
+  test('next() called with dev as array should send warning', () => {
+    const consoleSpy = jest.spyOn(console, 'warn')
+    const dev = ['array']
+    next({ dev, dir })
+
+    expect(consoleSpy).toHaveBeenCalledWith(warningMessage)
+  })
+
+  test('next() called with dev as object should send warning', () => {
+    const consoleSpy = jest.spyOn(console, 'warn')
+    const dev = { test: 'goes here' }
+    next({ dev, dir })
+
+    expect(consoleSpy).toHaveBeenCalledWith(warningMessage)
+  })
+
+  test('next() called with dev as function should send warning', () => {
+    const consoleSpy = jest.spyOn(console, 'warn')
+    const dev = () => console.log('test')
+    next({ dev, dir })
+
+    expect(consoleSpy).toHaveBeenCalledWith(warningMessage)
+  })
+})