Board Manager Update results in java.lang.NullPointerException

IDE 1.8.13, Platform linux64, Ubuntu 20.04.1
Have not be able to update boards using board manager. This error is the same as reported #10335 (and closed) last year. The apparent solution does not appear to be robust, since it has reoccured. I cannot update the Arduino SAMD board support package. Currently at 1.8.9. Clicking update results in the error.

```
java.lang.NullPointerException
java.lang.RuntimeException: java.lang.NullPointerException
at cc.arduino.contributions.packages.ui.ContributionManagerUI.lambda$onInstallPressed$2(ContributionManagerUI.java:175)
at java.lang.Thread.run(Thread.java:748)
Caused by: java.lang.NullPointerException
at cc.arduino.contributions.packages.ContributionInstaller.remove(ContributionInstaller.java:276)
at cc.arduino.contributions.packages.ui.ContributionManagerUI.lambda$onInstallPressed$2(ContributionManagerUI.java:170)
... 1 more
```

Is the solution still to delete .arduino15/cache/cache.json and ./arduino15/package_index.json and restart the Arduino IDE, as stated by facchinm on June 10, 2020?

Start at ContributionManagerUI.java lines 170-175 and ContributionInstaller.java line 276, then reproduce the Board Manager update for the Arduino SAMD package from 1.8.9. Check whether clearing .arduino15/cache/cache.json and ./arduino15/package_index.json changes the result. Done means the package update completes without a NullPointerException.

Written by the indexing model from the issue text.