fix: Check for null result in error handling (deephaven#5808)

Fixes deephaven#5806

engine/table/src/main/java/io/deephaven/engine/table/impl/MergedListener.java

@@ -162,7 +162,7 @@ protected void propagateError(
  }
 
  protected boolean systemicResult() {
- return SystemicObjectTracker.isSystemic(result);
+ return result == null ? false : SystemicObjectTracker.isSystemic(result);
  }
 
  @Override
@@ -172,6 +172,9 @@ protected void destroy() {
 
  protected void propagateErrorDownstream(
  final boolean fromProcess, @NotNull final Throwable error, @Nullable final TableListener.Entry entry) {
+ if (result == null) {
+ return;
+ }
  if (fromProcess && result.satisfied(getUpdateGraph().clock().currentStep())) {
  // If the result is already satisfied (because it managed to send its notification, or was otherwise
  // satisfied) we should not send our error notification on this cycle.