fix NPE on empty field conversion

CHANGELOG.md

@@ -37,6 +37,7 @@ Note that this project **does not** adhere to [Semantic Versioning](http://semve
 
 ### Fixed
 
+- We fixed an issue where converting empty fields led to an exception [#9200](https://github.com/JabRef/jabref/issues/9200)
 - We fixed an issue where author names with tilde accents (for example ñ) were marked as "Names are not in the standard BibTex format" [#8071](https://github.com/JabRef/jabref/issues/8071)
 - We fixed an issue where the possibility to generate a subdatabase from an aux file was writing empty files when called from the commandline [#9115](https://github.com/JabRef/jabref/issues/9115), [forum#3516](https://discourse.jabref.org/t/export-subdatabase-from-aux-file-on-macos-command-line/3516)
 - We fixed the display of issue, number, eid and pages fields in the entry preview. [#8607](https://github.com/JabRef/jabref/pull/8607), [#8372](https://github.com/JabRef/jabref/issues/8372), [Koppor#514](https://github.com/koppor/jabref/issues/514), [forum#2390](https://discourse.jabref.org/t/unable-to-edit-my-bibtex-file-that-i-used-before-vers-5-1/2390), [forum#3462](https://discourse.jabref.org/t/jabref-5-6-need-help-with-export-from-jabref-to-microsoft-word-entry-preview-of-apa-7-not-rendering-correctly/3462)

src/main/java/org/jabref/logic/formatter/bibtexfields/HtmlToLatexFormatter.java

@@ -26,7 +26,7 @@ public class HtmlToLatexFormatter extends Formatter implements LayoutFormatter {
 
     @Override
     public String format(String text) {
-        String result = Objects.requireNonNull(text);
+        String result = Objects.requireNonNullElse(text, "");
 
         if (result.isEmpty()) {
             return result;

src/main/java/org/jabref/logic/formatter/bibtexfields/HtmlToUnicodeFormatter.java

@@ -7,6 +7,8 @@
 
 import org.apache.commons.lang3.StringEscapeUtils;
 
+import java.util.Objects;
+
 @ApacheCommonsLang3Allowed("There is no equivalent in Google's Guava")
 public class HtmlToUnicodeFormatter extends Formatter implements LayoutFormatter {
 
@@ -33,6 +35,7 @@ public String getExampleInput() {
     @Override
     public String format(String fieldText) {
         // StringEscapeUtils converts characters and regex kills tags
-        return StringEscapeUtils.unescapeHtml4(fieldText).replaceAll("<[^>]*>", "");
+        String result = Objects.requireNonNullElse(fieldText, "");
+        return StringEscapeUtils.unescapeHtml4(result).replaceAll("<[^>]*>", "");
     }
 }

src/main/java/org/jabref/logic/formatter/bibtexfields/UnicodeToLatexFormatter.java

@@ -17,8 +17,7 @@ public class UnicodeToLatexFormatter extends Formatter implements LayoutFormatte
 
     @Override
     public String format(String text) {
-        String result = Objects.requireNonNull(text);
-
+        String result = Objects.requireNonNullElse(text, "");
         if (result.isEmpty()) {
             return result;
         }

src/main/java/org/jabref/model/strings/LatexToUnicodeAdapter.java

@@ -26,8 +26,8 @@ public class LatexToUnicodeAdapter {
      * @return a String with LaTeX resolved into Unicode, or the original String if the LaTeX could not be parsed
      */
     public static String format(String inField) {
-        Objects.requireNonNull(inField);
-        return parse(inField).orElse(Normalizer.normalize(inField, Normalizer.Form.NFC));
+        String result = Objects.requireNonNullElse(inField, "");
+        return parse(result).orElse(Normalizer.normalize(result, Normalizer.Form.NFC));
     }
 
     /**

src/test/java/org/jabref/logic/formatter/bibtexfields/HtmlToLatexFormatterTest.java

@@ -39,6 +39,8 @@ public void testBasic() {
 
     @Test
     public void testHTML() {
+        assertEquals("", formatter.format(null));
+        assertEquals("", formatter.format(""));
         assertEquals("{\\\"{a}}", formatter.format("ä"));
         assertEquals("{\\\"{a}}", formatter.format("ä"));
         assertEquals("{\\\"{a}}", formatter.format("ä"));

src/test/java/org/jabref/logic/formatter/bibtexfields/HtmlToUnicodeFormatterTest.java

@@ -15,6 +15,8 @@ public class HtmlToUnicodeFormatterTest {
 
     private static Stream<Arguments> data() {
         return Stream.of(
+                         Arguments.of("", ""),
+                         Arguments.of("", null),
                          Arguments.of("abc", "abc"),
                          Arguments.of("åäö", "&aring;&auml;&ouml;"),
                          Arguments.of("í", "i&#x301;"),

src/test/java/org/jabref/logic/formatter/bibtexfields/UnicodeToLatexFormatterTest.java

@@ -20,6 +20,7 @@ void setUp() {
 
     private static Stream<Arguments> testCases() {
         return Stream.of(
+                         Arguments.of("", null), // empty string input
                          Arguments.of("", ""), // empty string input
                          Arguments.of("abc", "abc"), // non unicode input
                          Arguments.of("{{\\aa}}{\\\"{a}}{\\\"{o}}", "\u00E5\u00E4\u00F6"), // multiple unicodes input

src/test/java/org/jabref/logic/layout/format/LatexToUnicodeFormatterTest.java

@@ -110,6 +110,11 @@ void keepUnknownCommandWithoutArgument() {
         assertEquals("\\aaaa", formatter.format("\\aaaa"));
     }
 
+    @Test
+    void keepUnknownCommandWithNullArgument() {
+        assertEquals("", formatter.format(null));
+    }
+
     @Test
     void keepUnknownCommandWithArgument() {
         assertEquals("\\aaaa{bbbb}", formatter.format("\\aaaa{bbbb}"));