Guard against a hypothetical null pointer dereference

pkg/analysis_server/lib/src/status/memory_use.dart

Index: pkg/analysis_server/lib/src/status/memory_use.dart
diff --git a/pkg/analysis_server/lib/src/status/memory_use.dart b/pkg/analysis_server/lib/src/status/memory_use.dart
index 92657bcdc0454cfa26f3bdda521d0e47d9632b0f..8f68b0f37bdee06d4b6ea4e8066a0fc3f521eff5 100644
--- a/pkg/analysis_server/lib/src/status/memory_use.dart
+++ b/pkg/analysis_server/lib/src/status/memory_use.dart
@@ -225,8 +225,10 @@ class MemoryUseData {
     DartSdkManager manager = server.sdkManager;
     List<SdkDescription> descriptors = manager.sdkDescriptors;
     for (SdkDescription descriptor in descriptors) {
-      _processAnalysisContext(
-          manager.getSdk(descriptor, () => null).context, manager);
+      DartSdk sdk = manager.getSdk(descriptor, () => null);
+      if (sdk != null) {
+        _processAnalysisContext(sdk.context, manager);
+      }
     }
   }