Make sure mainContainer and owner are not null

diff --git a/src/jalview/appletgui/Tooltip.java b/src/jalview/appletgui/Tooltip.java
index 90f2e97..7de5193 100755 (executable)
--- a/src/jalview/appletgui/Tooltip.java
+++ b/src/jalview/appletgui/Tooltip.java
@@ -137,6 +137,10 @@ public class Tooltip
 \r
   void setTipLocation(MouseEvent evt)\r
   {\r
+    if(mainContainer==null || owner==null)\r
+    {\r
+      return;\r
+    }\r
     setLocation( (owner.getLocationOnScreen().x\r
                   - mainContainer.getLocationOnScreen().x) + evt.getX(),\r
                 (owner.getLocationOnScreen().y -\r