Enable null checking for SplittablepairWithRestrictionDoFnRunner

Author: kennknowles

sdks/java/harness/src/main/java/org/apache/beam/fn/harness/SplittablePairWithRestrictionDoFnRunner.java

@@ -58,9 +58,6 @@
  * WindowedValue<InputT>} to {@code WindowedValue<KV<InputT, KV<RestrictionT,
  * WatermarkEstimatorStateT>>>}
  */
-@SuppressWarnings({
-  "nullness" // TODO(https://github.com/apache/beam/issues/20497)
-})
 public class SplittablePairWithRestrictionDoFnRunner<
         InputT, RestrictionT, WatermarkEstimatorStateT, OutputT>
     implements FnApiStateAccessor.MutatingStateContext<Object, BoundedWindow> {
@@ -309,7 +306,11 @@ public BoundedWindow window() {
 
     @Override
     public Object sideInput(String tagId) {
-      return stateAccessor.get(sideInputMapping.get(tagId), getCurrentWindowOrFail());
+      PCollectionView<Object> pCollectionView =
+          (PCollectionView<Object>)
+              checkStateNotNull(sideInputMapping.get(tagId), "Side input tag not found: %s", tagId);
+
+      return stateAccessor.get(pCollectionView, getCurrentWindow());
     }
   }