code: @Override
      public void sawOpcode(int seen) {
          try {
  
              stack.precomputation(this);
  
              if ((seen == Const.ARETURN) && (stack.getStackDepth() > 0)) {
                  OpcodeStack.Item item = stack.getStackItem(0);
                  if (item.isNull()) {
                      bugReporter.reportBug(new BugInstance(this, BugType.TBP_TRISTATE_BOOLEAN_PATTERN.name(), NORMAL_PRIORITY).addClass(this).addMethod(this)
                              .addSourceLine(this));
                      throw new StopOpcodeParsingException();
                  }
              }
          } finally {
              stack.sawOpcode(this, seen);
          }
      }
docs: implements the visitor to look for null returns