Strip invalid prolog when loading CitationStyles (JabRef#3404)

CHANGELOG.md

@@ -42,6 +42,7 @@ We refer to [GitHub issues](https://github.com/JabRef/jabref/issues) by using `#
  - We fixed several issues with the automatic linking of files in the entry editor where files were not found or not correctly saved in the bibtex source [#3346](https://github.com/JabRef/jabref/issues/3346)
  - We fixed an issue where fetching entries from crossref that had no titles caused an error [#3376](https://github.com/JabRef/jabref/issues/3376)
  - We fixed an issue where the same Java Look and Feel would be listed more than once in the Preferences. [#3391](https://github.com/JabRef/jabref/issues/3391)
+ - We fixed an issue where errors in citation styles triggered an exception when opening the preferences dialog [#3389](https://github.com/JabRef/jabref/issues/3389)
 
  ### Removed
 

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

@@ -58,11 +58,11 @@ private CitationStyle(final String filename, final String title, final String so
      * Creates an CitationStyle instance out of the style string
      */
     private static CitationStyle createCitationStyleFromSource(final String source, final String filename) {
-        if (source != null && !source.isEmpty() && filename != null && !filename.isEmpty()) {
+        if (filename != null && !filename.isEmpty() && source != null && !source.isEmpty()) {
             try {
                 DocumentBuilder db = DocumentBuilderFactory.newInstance().newDocumentBuilder();
                 InputSource is = new InputSource();
-                is.setCharacterStream(new StringReader(source));
+                is.setCharacterStream(new StringReader(stripInvalidProlog(source)));
 
                 Document doc = db.parse(is);
                 NodeList nodes = doc.getElementsByTagName("info");
@@ -78,6 +78,15 @@ private static CitationStyle createCitationStyleFromSource(final String source,
         return null;
     }
 
+    private static String stripInvalidProlog(String source) {
+        int startIndex = source.indexOf("<");
+        if (startIndex > 0) {
+            return source.substring(startIndex, source.length());
+        } else {
+            return source;
+        }
+    }
+
     /**
      * Loads the CitationStyle from the given file
      */