Fix a null pointer exception

pkg/analysis_server/lib/src/computer/computer_hover.dart

Index: pkg/analysis_server/lib/src/computer/computer_hover.dart
diff --git a/pkg/analysis_server/lib/src/computer/computer_hover.dart b/pkg/analysis_server/lib/src/computer/computer_hover.dart
index 213f16ed72ad2dfd0af3a19be7bfed93e8abb295..87bc87a4bd808d9f35cdc77b5a2435fcf8ed0ce2 100644
--- a/pkg/analysis_server/lib/src/computer/computer_hover.dart
+++ b/pkg/analysis_server/lib/src/computer/computer_hover.dart
@@ -68,6 +68,9 @@ class DartUnitHoverComputer {
    */
   HoverInformation compute() {
     AstNode node = new NodeLocator.con1(_offset).searchWithin(_unit);
+    if (node == null) {
+      return null;
+    }
     if (node.parent is TypeName &&
         node.parent.parent is ConstructorName &&
         node.parent.parent.parent is InstanceCreationExpression) {