fix for cancel write to file

git-svn-id: svn+ssh://svn.internal.sanger.ac.uk/repos/svn/pathsoft/artemis/trunk@15621 ee4ac58c-ac51-4696-9907-e4b3aa274f04

uk/ac/sanger/artemis/components/variant/IOUtils.java

@@ -296,6 +296,8 @@ class IOUtils
       if(!view)
       {
         File f = getFile(vcfReaders[0].getFileName(), 1, suffix, yBox);
+        if(f == null)
+          return;
         writer = new FileWriter(f);
         fastaFiles += f.getAbsolutePath()+"\n";
       }