Show dialog when copy files did not found file (#3826)

Follow up from #3818

Author: Siedlerchr

src/main/java/org/jabref/gui/copyfiles/CopyFilesAction.java

@@ -63,6 +63,10 @@ private void startServiceAndshowProgessDialog(Task<List<CopyFilesResultItemViewM
     }
 
     private void showDialog(List<CopyFilesResultItemViewModel> data) {
+        if (data.isEmpty()) {
+            dialogService.showInformationDialogAndWait(Localization.lang("Copy linked files to folder..."), Localization.lang("No linked files found for export."));
+            return;
+        }
         CopyFilesDialogView dlg = new CopyFilesDialogView(databaseContext, new CopyFilesResultListDependency(data));
         dlg.show();
     }

src/main/resources/l10n/JabRef_en.properties

@@ -2350,3 +2350,5 @@ Aux\ file=Aux file
 Group\ containing\ entries\ cited\ in\ a\ given\ TeX\ file=Group containing entries cited in a given TeX file
 
 Any\ file=Any\ file
+
+No\ linked\ files\ found\ for\ export.=No linked files found for export.