What is the cause of the connection error in the H2 database client embedded in IntelliJ idea, stating that the database may already be in use with a null value?

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There are several potential causes for this error:

1. The H2 database may already be running in another process, which means that IntelliJ IDEA cannot connect to the database because it is already in use.

2. The database file may be locked by another process, preventing IntelliJ IDEA from accessing it.

3. There could be a problem with the connection settings in IntelliJ IDEA, such as incorrect credentials or an incorrect URL.

4. There could be a problem with the H2 database itself, such as a corrupted file or a malfunctioning driver.

To troubleshoot this issue, you can try the following steps:

1. Make sure that no other processes are using the H2 database file.

2. Check the connection settings in IntelliJ IDEA to make sure they are correct.

3. Try connecting to a different H2 database to see if the problem persists.

4. If none of these steps resolves the issue, try reinstalling the H2 database driver and restarting IntelliJ IDEA.