NPE JDBCUpsertOutputFormat

Encountered a situation where I get an NPE from JDBCUpsertOutputFormat.
This occurs when close is called before open.

This happened because I had a sink where it had a final field of type JDBCUpsertOutputFormat.

The open operation of my sink was slow (blocked on something else) and open on the JDBCUpsertOutputFormat had not yet been called.
In the mean time the job was cancelled, which caused close on my sink to be called, which then
called close on the JDBCUpsertOutputFormat .
This throws an NPE due to a lack of a guard on an internal field that is only initialised in the JDBCUpsertOutputFormat open operation.

The close method already guards one potentially null value ..

if (this.scheduledFuture != null) {

But needs the additional guard below ...

if (jdbcWriter != null)   // << THIS LINE NEEDED TO GUARD UNINITIALISE VAR
   try {
      jdbcWriter.close();
   } catch (SQLException e) {
      LOG.warn("Close JDBC writer failed.", e);
   }

See also FLINK-17545