Fix action helper when no library is open

Author: Siedlerchr

src/main/java/org/jabref/gui/actions/ActionHelper.java

@@ -7,6 +7,7 @@
 
 import javafx.beans.binding.Bindings;
 import javafx.beans.binding.BooleanExpression;
+import javafx.beans.property.SimpleStringProperty;
 import javafx.collections.ObservableList;
 
 import org.jabref.gui.StateManager;
@@ -51,6 +52,10 @@ public static BooleanExpression isAnyFieldSetForSelectedEntry(List<Field> fields
 
     public static BooleanExpression isFilePresentForSelectedEntry(StateManager stateManager, PreferencesService preferencesService) {
         return Bindings.createBooleanBinding(() -> {
+
+                    if (stateManager.getSelectedEntries().isEmpty()) {
+                        return false;
+                    }
                     List<LinkedFile> files = stateManager.getSelectedEntries().get(0).getFiles();
                     if ((files.size() > 0) && stateManager.getActiveDatabase().isPresent()) {
                         Optional<Path> filename = FileHelper.find(
@@ -62,6 +67,6 @@ public static BooleanExpression isFilePresentForSelectedEntry(StateManager state
                         return false;
                     }
                 }, stateManager.getSelectedEntries(),
-                stateManager.getSelectedEntries().get(0).getFieldBinding(StandardField.FILE));
+                stateManager.getSelectedEntries().isEmpty() ? new SimpleStringProperty("") : stateManager.getSelectedEntries().get(0).getFieldBinding(StandardField.FILE));
     }
 }

src/main/java/org/jabref/gui/documentviewer/ShowDocumentViewerAction.java

@@ -5,10 +5,12 @@
 import org.jabref.gui.actions.SimpleCommand;
 import org.jabref.preferences.PreferencesService;
 
+import static org.jabref.gui.actions.ActionHelper.needsEntriesSelected;
+
 public class ShowDocumentViewerAction extends SimpleCommand {
 
     public ShowDocumentViewerAction(StateManager stateManager, PreferencesService preferences) {
-        this.executable.bind(ActionHelper.isFilePresentForSelectedEntry(stateManager, preferences));
+        this.executable.bind(needsEntriesSelected(stateManager).and(ActionHelper.isFilePresentForSelectedEntry(stateManager, preferences)));
     }
 
     @Override