Band name with "/" for netCDF output throws NPE

This process graph will throw a NPE and it's not immediately apparent what causes it:

{
  "process_graph": {
    "applydimension1": {
      "arguments": {
        "data": {
          "from_node": "loadcollection1"
        },
        "dimension": "bands",
        "process": {
          "process_graph": {
            "arrayappend1": {
              "arguments": {
                "data": {
                  "from_parameter": "data"
                },
                "value": {
                  "from_node": "divide1"
                }
              },
              "process_id": "array_append",
              "result": true
            },
            "arrayelement1": {
              "arguments": {
                "data": {
                  "from_parameter": "data"
                },
                "index": 1
              },
              "process_id": "array_element"
            },
            "arrayelement2": {
              "arguments": {
                "data": {
                  "from_parameter": "data"
                },
                "index": 0
              },
              "process_id": "array_element"
            },
            "divide1": {
              "arguments": {
                "x": {
                  "from_node": "arrayelement1"
                },
                "y": {
                  "from_node": "arrayelement2"
                }
              },
              "process_id": "divide"
            }
          }
        }
      },
      "process_id": "apply_dimension"
    },
    "loadcollection1": {
      "arguments": {
        "bands": [
          "VV",
          "VH"
        ],
        "id": "SENTINEL1_GLOBAL_MOSAICS",
        "spatial_extent": {
          "crs": "EPSG:32755",
          "east": 576780,
          "north": 5246260,
          "south": 5244980,
          "west": 575500
        },
        "temporal_extent": [
          "2019-12-31T23:59:59Z",
          "2021-01-01T00:00:00Z"
        ]
      },
      "process_id": "load_collection"
    },
    "renamelabels1": {
      "arguments": {
        "data": {
          "from_node": "applydimension1"
        },
        "dimension": "bands",
        "target": [
          "VV",
          "VH",
          "VH/VV"
        ]
      },
      "process_id": "rename_labels"
    },
    "saveresult1": {
      "arguments": {
        "data": {
          "from_node": "renamelabels1"
        },
        "format": "NetCDF",
        "options": {
          "filename_prefix": "lcfm-s1-55GEN_062_19"
        }
      },
      "process_id": "save_result",
      "result": true
    }
  }
}

Traceback (most recent call last):
  File "/opt/openeo/lib/python3.8/site-packages/openeogeotrellis/deploy/batch_job.py", line 518, in start_main
    main(sys.argv)
  File "/opt/openeo/lib/python3.8/site-packages/openeogeotrellis/deploy/batch_job.py", line 194, in main
    run_driver()
  File "/opt/openeo/lib/python3.8/site-packages/openeogeotrellis/deploy/batch_job.py", line 165, in run_driver
    run_job(
  File "/opt/openeo/lib/python3.8/site-packages/openeogeotrellis/utils.py", line 56, in memory_logging_wrapper
    return function(*args, **kwargs)
  File "/opt/openeo/lib/python3.8/site-packages/openeogeotrellis/deploy/batch_job.py", line 315, in run_job
    the_assets_metadata = result.write_assets(str(output_file))
  File "/opt/openeo/lib/python3.8/site-packages/openeo_driver/save_result.py", line 150, in write_assets
    return self.cube.write_assets(filename=directory, format=self.format, format_options=self.options)
  File "/opt/openeo/lib/python3.8/site-packages/openeogeotrellis/geopysparkdatacube.py", line 1966, in write_assets
    asset_paths = get_jvm().org.openeo.geotrellis.netcdf.NetCDFRDDWriter.writeRasters(
  File "/usr/local/spark/python/lib/py4j-0.10.9.7-src.zip/py4j/java_gateway.py", line 1322, in __call__
    return_value = get_return_value(
  File "/usr/local/spark/python/lib/py4j-0.10.9.7-src.zip/py4j/protocol.py", line 326, in get_return_value
    raise Py4JJavaError(
py4j.protocol.Py4JJavaError: An error occurred while calling z:org.openeo.geotrellis.netcdf.NetCDFRDDWriter.writeRasters.
: java.lang.NullPointerException
	at ucar.nc2.jni.netcdf.Nc4Iosp.writeData(Nc4Iosp.java:2986)
	at ucar.nc2.NetcdfFileWriter.write(NetcdfFileWriter.java:1066)
	at ucar.nc2.NetcdfFileWriter.write(NetcdfFileWriter.java:1050)
	at org.openeo.geotrellis.netcdf.NetCDFRDDWriter$.writeTile(NetCDFRDDWriter.scala:308)
	at org.openeo.geotrellis.netcdf.NetCDFRDDWriter$.$anonfun$saveSingleNetCDFGeneric$5(NetCDFRDDWriter.scala:217)
	at scala.collection.immutable.Range.foreach$mVc$sp(Range.scala:158)
	at org.openeo.geotrellis.netcdf.NetCDFRDDWriter$.$anonfun$saveSingleNetCDFGeneric$3(NetCDFRDDWriter.scala:195)
	at org.openeo.geotrellis.netcdf.NetCDFRDDWriter$.$anonfun$saveSingleNetCDFGeneric$3$adapted(NetCDFRDDWriter.scala:158)
	at scala.collection.Iterator.foreach(Iterator.scala:943)
	at scala.collection.Iterator.foreach$(Iterator.scala:943)
	at scala.collection.AbstractIterator.foreach(Iterator.scala:1431)
	at org.openeo.geotrellis.netcdf.NetCDFRDDWriter$.saveSingleNetCDFGeneric(NetCDFRDDWriter.scala:158)
	at org.openeo.geotrellis.netcdf.NetCDFRDDWriter$.saveSingleNetCDFGeneric(NetCDFRDDWriter.scala:108)
	at org.openeo.geotrellis.netcdf.NetCDFRDDWriter$.writeRasters(NetCDFRDDWriter.scala:80)
	at org.openeo.geotrellis.netcdf.NetCDFRDDWriter.writeRasters(NetCDFRDDWriter.scala)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at py4j.reflection.MethodInvoker.invoke(MethodInvoker.java:244)
	at py4j.reflection.ReflectionEngine.invoke(ReflectionEngine.java:374)
	at py4j.Gateway.invoke(Gateway.java:282)
	at py4j.commands.AbstractCommand.invokeMethod(AbstractCommand.java:132)
	at py4j.commands.CallCommand.execute(CallCommand.java:79)
	at py4j.ClientServerConnection.waitForCommands(ClientServerConnection.java:182)
	at py4j.ClientServerConnection.run(ClientServerConnection.java:106)
	at java.base/java.lang.Thread.run(Thread.java:829)

The same process graph works fine for e.g. GeoTiff.

Maybe this one-way replace has something to do with it?

https://github.com/Open-EO/openeo-geotrellis-extensions/blob/9782c10f65510f0ff7214e15bb573c701f6afb6f/openeo-geotrellis/src/main/scala/org/openeo/geotrellis/netcdf/NetCDFRDDWriter.scala#L692

Possible courses of action:

• silently replace illegal characters but make sure to avoid this error further on;
• throw a clearer error.