File Annoation Tab gives NPE when PDF is not avaiable
Entry with linked PDF:
` file          = {:Arellano2015 - Natural Language Processing of Textual Requirements.pdf:PDF},`
but the PDF is not available/existing in the folder.

Switch to Tab File Annotations: NPE exception comes:
```
java.lang.NullPointerException: null
	at org.jabref.gui.entryeditor.FileAnnotationTab.updateShownAnnotations(FileAnnotationTab.java:130) ~[bin/:?]
	at org.jabref.gui.entryeditor.FileAnnotationTab.addAnnotations(FileAnnotationTab.java:114) ~[bin/:?]
	at org.jabref.gui.entryeditor.FileAnnotationTab.initializeTab(FileAnnotationTab.java:89) ~[bin/:?]
	at org.jabref.gui.entryeditor.EntryEditor$TabListener.lambda$0(EntryEditor.java:1155) ~[bin/:?]
	at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311) ~[?:1.8.0_121]
```
diff --git a/src/main/java/org/jabref/gui/entryeditor/FileAnnotationTab.java b/src/main/java/org/jabref/gui/entryeditor/FileAnnotationTab.java
index 7c6901c310..14483cec7b 100644
--- a/src/main/java/org/jabref/gui/entryeditor/FileAnnotationTab.java
+++ b/src/main/java/org/jabref/gui/entryeditor/FileAnnotationTab.java
@@ -127,7 +127,7 @@ class FileAnnotationTab extends JPanel {
      */
     private void updateShownAnnotations(List<FileAnnotation> annotations) {
         listModel.clear();
-        if (annotations.isEmpty()) {
+        if (annotations == null || annotations.isEmpty()) {
             listModel.addElement(new FileAnnotation("", LocalDateTime.now(), 0, Localization.lang("File has no attached annotations"), NONE, Optional.empty()));
         } else {
             Comparator<FileAnnotation> byPage = Comparator.comparingInt(FileAnnotation::getPage);
['src/main/java/org/jabref/gui/entryeditor/FileAnnotationTab.java']