Fix Copy linked files dialog showing NPE when no linked files

CHANGELOG.md

@@ -51,6 +51,7 @@ The new default removes the linked file from the entry instead of deleting the f
 - We fixed an issue where not all bibtex/biblatex fields would be exported as latex-free to MS-Office XML [koppor#284](https://github.com/koppor/jabref/issues/284)
 - We fixed an issue where linked files would be deleted from bibliography entries despite choosing the "Cancel" option in the dialog menu.
 - We fixed the name of the group editing window to "Add group" instead of "Edit Group" when adding a new group. [koppor#277](https://github.com/koppor/jabref/issues/277)
+- We fixed an issue where the "Copy linked files" dialog produced an error when the entry had no file [#3818](https://github.com/JabRef/jabref/issues/3818)
 
 ### Removed
 - We removed the [Look and Feels from JGoodies](http://www.jgoodies.com/freeware/libraries/looks/), because the open source version is not compatible with Java 9.

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

@@ -52,10 +52,14 @@ public void actionPerformed(ActionEvent e) {
 
     private void startServiceAndshowProgessDialog(Task<List<CopyFilesResultItemViewModel>> exportService) {
 
-        DefaultTaskExecutor.runInJavaFXThread(() -> dialogService.showCanceableProgressDialogAndWait(exportService));
+        DefaultTaskExecutor.runInJavaFXThread(() -> {
+            dialogService.showCanceableProgressDialogAndWait(exportService);
+        });
 
-        exportService.run(); //Run kinda blocks, so we just show the result dialog wgeb run is ready
-        DefaultTaskExecutor.runInJavaFXThread(() -> showDialog(exportService.getValue()));
+        exportService.run();
+        DefaultTaskExecutor.runInJavaFXThread(() -> {
+            showDialog(exportService.getValue());
+        });
     }
 
     private void showDialog(List<CopyFilesResultItemViewModel> data) {

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

@@ -38,7 +38,7 @@ public class CopyFilesTask extends Task<List<CopyFilesResultItemViewModel>> {
     private final long totalFilesCount;
     private final List<BibEntry> entries;
     private final List<CopyFilesResultItemViewModel> results = new ArrayList<>();
-    private Optional<Path> newPath;
+    private Optional<Path> newPath = Optional.empty();
     private int numberSucessful;
     private int totalFilesCounter;
 
@@ -54,8 +54,7 @@ public CopyFilesTask(BibDatabaseContext databaseContext, List<BibEntry> entries,
     }
 
     @Override
-    protected List<CopyFilesResultItemViewModel> call()
-            throws InterruptedException, IOException {
+    protected List<CopyFilesResultItemViewModel> call() throws InterruptedException, IOException {
 
         updateMessage(Localization.lang("Copying files..."));
         updateProgress(0, totalFilesCount);
@@ -103,6 +102,7 @@ protected List<CopyFilesResultItemViewModel> call()
                 }
             }
             updateMessage(Localization.lang("Finished copying"));
+
             String sucessMessage = Localization.lang("Copied %0 files of %1 sucessfully to %2", Integer.toString(numberSucessful), Integer.toString(totalFilesCounter), newPath.map(Path::getParent).map(Path::toString).orElse(""));
             updateMessage(sucessMessage);
             bw.write(sucessMessage);