Check if assists are requested on a invalid offset.

pkg/analysis_server/lib/src/services/correction/assist_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/assist_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/assist_internal.dart b/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
index 84eeeac1a476b25350b5bdecc42563b47b4c9f8d..e2b74663a46c382e7e220fbba6d738adc0a877c7 100644
--- a/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/assist_internal.dart
@@ -76,6 +76,9 @@ class AssistProcessor {
     utils = new CorrectionUtils(unit);
     node =
         new NodeLocator.con2(selectionOffset, selectionEnd).searchWithin(unit);
+    if (node == null) {
+      return assists;
+    }
     // try to add proposals
     _addProposal_addTypeAnnotation_DeclaredIdentifier();
     _addProposal_addTypeAnnotation_VariableDeclaration();