Fix for NPE in navigation after some incremental changes.

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

Index: pkg/analysis_server/lib/src/computer/computer_navigation.dart
diff --git a/pkg/analysis_server/lib/src/computer/computer_navigation.dart b/pkg/analysis_server/lib/src/computer/computer_navigation.dart
index ab0d8547f53b5cb462b1c4da2deb29dcb1c06b02..0c14dd699aa22a0cd36e5b8e3fa7b47a377d3e36 100644
--- a/pkg/analysis_server/lib/src/computer/computer_navigation.dart
+++ b/pkg/analysis_server/lib/src/computer/computer_navigation.dart
@@ -44,6 +44,11 @@ class DartUnitNavigationComputer {
       return;
     }
     int targetIndex = _addTarget(element);
+    // TODO(scheglov) Fix the enclosing element problem in the incremental
+    // resolver and remove this.
+    if (targetIndex == null) {
+      return;
+    }
     regions.add(
         new protocol.NavigationRegion(offset, length, <int>[targetIndex]));
   }
@@ -54,6 +59,11 @@ class DartUnitNavigationComputer {
       index = targets.length;
       protocol.NavigationTarget target =
           protocol.newNavigationTarget_fromElement(element, _addFile);
+      // TODO(scheglov) Fix the enclosing element problem in the incremental
+      // resolver and remove this.
+      if (target == null) {
+        return null;
+      }
       targets.add(target);
       targetMap[element] = index;
     }