Addresses #11521 (comment)
If, in a development setup, csl-styles submodules are missing, NullPointerException is thrown when trying to access the default (ieee) style.

• The try-catch block in context:
  

  jabref/src/main/java/org/jabref/logic/citationstyle/CitationStyle.java

  Lines 120 to 128 in 771c4cd

  	try (InputStream inputStream = isExternalFile ? Files.newInputStream(internalFilePath) : CitationStyle.class.getResourceAsStream(internalFile)) {
  	return createCitationStyleFromSource(inputStream, styleFile);
  	} catch (NoSuchFileException e) {
  	LOGGER.error("Could not find file: {}", styleFile, e);
  	} catch (IOException e) {
  	LOGGER.error("Error reading source file", e);
  	}
  	return Optional.empty();
  	}

  
  is using try-with-resources, which automatically closes the InputStream. However, if getResourceAsStream returns null, no exception is thrown at this point. The null InputStream is passed to createCitationStyleFromSource. This has been modified to add a null check.
• A null pointer exception has also been additionally caught and handled at the callee function createCitationStyleFromSource.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.