NetBeans 8.2 does not support Java 9. It just won't work.
The solution is to download the latest nightly build of NetBeans, which does support Java 9:
Normally you would go to the NetBeans download page and click the Development link in the top right corner of the screen.
However, I just tried that and it looks like their web page for downloading nightly builds is not working properly at the moment due to "...violates the following Content Security Policy directive..." errors. I've never seen that before.
A temporary workaround for that problem is to go directly to Index of /download/trunk/nightly/latest/bundles/ and select the download that is appropriate for your environment. You should be able to parse the file names to work out which one you want, depending on whether you need 32-bit or 64-bit, whether you are running on Mac OS or Linux or Windows, etc. (To be clear, ignore this bullet point if the Development link in the first bullet point above is working properly.)
Also note that you can safely install and run multiple versions of NetBeans concurrently: use version 8.2 for Java 8 development, and use the Development Version of NetBeans for Java 9 development.
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Update:
The page for downloading Development Builds of NetBeans is working again, so go there, select your Platform from the droplist, and then click the appropriate Download button.
NetBeans with Java 9 is working fine for me using that approach.