Bug report

Describe the bug

Using a basePath different from null on the webpack config makes the sourcemaps plugin fail

To Reproduce

Steps to reproduce the behavior:

1. git clone git@github.com:Agupane/next.js-1.git
2. Install the dependencies
3. Setup the env variables for a sentry project:

NEXT_PUBLIC_SENTRY_DSN=
SENTRY_ORG
SENTRY_PROJECT=
SENTRY_AUTH_TOKEN=
VERCEL_GITHUB_COMMIT_SHA=
COMMIT_SHA=

4. Go to http://localhost:3000
5. Click on the button This button will make the app throw an error
6. Go to sentry and see that the sourcemaps are not displayed

Expected behavior

I expect to be able to change the default basePath of webpack and see the sourcemaps on sentry

Screenshots

With basePath different from empty:

With empty basePath:

System information

• OS: macOS
• Browser: Chrome
• Version of Next.js: 9.5.2
• Version of Node.js: 10.16.3

Additional context

Add any other context about the problem here.