Fix options databinding by adding elements if they are not in the initial map

CHANGELOG.md

@@ -63,6 +63,7 @@ We refer to [GitHub issues](https://github.com/JabRef/jabref/issues) by using `#
  - We fixed an issue where dropping a pdf on the entry table and renaming it triggered an exception [#3490](https://github.com/JabRef/jabref/issues/3490)
  - We fixed an issue where integrating external changes to a bib file caused instability [#3498](https://github.com/JabRef/jabref/issues/3498)
  - We fixed an issue where fetched entries from the ACM fetcher could not be imported [#3500](https://github.com/JabRef/jabref/issues/3500)
+ - We fixed an issue where custom data in combobox fields in the entry editor was not saved [#3538](https://github.com/JabRef/jabref/issues/3538)
 
  ### Removed
 

src/main/java/org/jabref/gui/fieldeditors/MapBasedEditorViewModel.java

@@ -9,12 +9,16 @@
 import org.jabref.logic.integrity.FieldCheckers;
 
 import com.google.common.collect.BiMap;
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 
 /**
  * View model for a field editor that shows various options backed by a map.
  */
 public abstract class MapBasedEditorViewModel<T> extends OptionEditorViewModel<T> {
 
+    private static final Log LOGGER = LogFactory.getLog(MapBasedEditorViewModel.class);
+
     public MapBasedEditorViewModel(String fieldName, AutoCompleteSuggestionProvider<?> suggestionProvider, FieldCheckers fieldCheckers) {
         super(fieldName, suggestionProvider, fieldCheckers);
     }
@@ -24,12 +28,13 @@ public MapBasedEditorViewModel(String fieldName, AutoCompleteSuggestionProvider<
     @Override
     public StringConverter<T> getStringConverter() {
         return new StringConverter<T>() {
+
             @Override
             public String toString(T object) {
                 if (object == null) {
                     return null;
                 } else {
-                    return getItemMap().inverse().getOrDefault(object, "");
+                    return getItemMap().inverse().getOrDefault(object, object.toString()); //if the object is not found we simply return itself as string
                 }
             }
 
@@ -38,12 +43,28 @@ public T fromString(String string) {
                 if (string == null) {
                     return null;
                 } else {
-                    return getItemMap().getOrDefault(string, null);
+                    return getItemMap().getOrDefault(string, getValueFromString(string));
                 }
             }
         };
     }
 
+    /**
+     * Converts a String value to the Type T. If the type cannot be directly casted to T, this method must be overriden in a subclass
+     * @param string The input value to convert
+     * @return The value or null if the value could not be casted
+     */
+    @SuppressWarnings("unchecked")
+    protected T getValueFromString(String string) {
+        try {
+            return (T) string;
+        } catch (ClassCastException ex) {
+            LOGGER.error(String.format("Could not cast string to type %1$s. Try overriding the method in a subclass and provide a conversion from string to the concrete type %1$s", string.getClass()), ex);
+        }
+        return null;
+
+    }
+
     @Override
     public List<T> getItems() {
         return new ArrayList<>(getItemMap().values());