Configure method in Predictor node

Hi guys, i have (another) problem: I'm developing a predictor node with 2 input ports (MyModel, BufferedDataTable) and 1 output port (BufferedDataTable). When i try to insert the node in a workflow i receive this message:
ERROR     PredictorDc                        Configure failed (NullPointerException): null

I search it in the forum, but all the answers are too specific for each case.
I understand the problem could be that the node try to configure itself as I insert it into the workflow and it has no inputs. But why i do not receive this message by others node wich do the same? (e.g. decisiontree)

Can anyone help me? How can i fix it? 
Here my code: (i follow the example on the site)

 /** {@inheritDoc} */
    protected PortObjectSpec[] configure(final PortObjectSpec[] inSpecs) throws InvalidSettingsException {
    	// produce the output table spec, 
        DataColumnSpec newColumnSpec = createOutputColumnSpec();
        // and the DataTableSpec for the appended part
        DataTableSpec appendedSpec = new DataTableSpec(newColumnSpec);
        // since it is only appended the new output spec contains both:
        // the original spec and the appended one
        DataTableSpec outputSpec = new DataTableSpec((DataTableSpec) inSpecs[IN_PORT_DATATABLE], appendedSpec);
        return new PortObjectSpec[]{outputSpec};
    	//return new PortObjectSpec[]{inSpecs[1]};
    private DataColumnSpec createOutputColumnSpec() {
        // we want to add a column with the infered result 
        DataColumnSpecCreator colSpecCreator = new DataColumnSpecCreator("Infered", StringCell.TYPE);
        // now the column spec can be created
        DataColumnSpec newColumnSpec = colSpecCreator.createSpec();
        return newColumnSpec;

P.S. I wrote the post with the help of Google Translate, so I apologize for any errors

The log file contains the complete stack trace of the NullPointerException. This helps you in identifying the problem.

Or add a NullPointerException breakpoint while debugging.