BufferedWriter throwing NullOutputStream on write

Question

I create a new ProcessBuilder as:

ProcessBuilder pb = new ProcessBuilder("java", "-Xmx1G", "-classpath", "<path_to_class>", "<class_name>");

pb.directory(new File(<insert_directory>));
pb.redirectErrorStream(true);
pb.environment().put("PATH","<env_path>");

Process p = pb.start();

Downstream a bit, I run:

Process p = Runtime.getRuntime().exect(<command>);
BufferedWriter input=new BufferedWriter(new OutputStreamWriter(p.getOutputStream()));
BufferedReader output=new BufferedReader(new InputStreamReader(p.getInputStream()));
BufferedReader error=new BufferedReader(new InputStreamReader(p.getErrorStream()));

input.write(<string>);

I can write to the output and error BufferedReaders. But I keep getting java.io.IOException: Stream closed on ProcessBuilder$NullOutputStream.write

Other solutions have suggested getting rid of .redirectIO(), but I'm not using that here.

I can comment out the failing input section and the rest of the code executes as expected.

Any suggestions?