Unable to use start/stop xwiki bash script after a reboot

Reproduction steps:

• Start XWiki using start_xwiki.sh on Linux
• Reboot (or maybe hard reboot?)
• Try again to start XWiki using start_xwiki.sh

Expected result:
The wiki start.

Actual result:
I get the following message:

An XWiki instance is already running on port 8080. Aborting...
Consider calling stop_xwiki.sh to stop it.

When calling stop_xwiki.sh, I get the following error:

Attempting to stop XWiki cleanly on port 8080...
java.lang.NullPointerException
at org.eclipse.jetty.start.Main.stop(Main.java:618)
at org.eclipse.jetty.start.Main.stop(Main.java:558)
at org.eclipse.jetty.start.Main.doStop(Main.java:546)
at org.eclipse.jetty.start.Main.start(Main.java:422)
at org.eclipse.jetty.start.Main.main(Main.java:78)

Usage: java -jar $JETTY_HOME/start.jar [options] [properties] [configs]
java -jar $JETTY_HOME/start.jar --help # for more information

The error seems related to the usage of a lock file inside the directory /var/tmp which is not cleaned by default when restarting the computer.
The error message I get when trying to run the Wiki is then wrong: I have nothing running on port 8080.