MATLAB, error while trying to implement a Designed Filter object
The issue has been resolved. This is due to the absence of the filter object fields:

                    FilterFunction:
                  FiltfiltFunction:
GetCoefficientsForAnalysisFunction:


In MATLAB 2015a these fields are absent from the filters objects. But in MATLAB 2016a (which is I am using, these fields seem to be required.) As the files were imported after creation in MATLAB 2015a, that seems to be the problem. Recreating, the fields in MATLAB 2016a solved the issue. The fields have also been created.

The code ran to check:

filt = D_sig50(1);
sf = struct(filt) ;
sf.FilterFunction


Created in MATLAB 2015a:
output:

                    FilterFunction: []
                  FiltfiltFunction: []
GetCoefficientsForAnalysisFunction: []


These fields in the filter object are empty (after loading these filters in MATLAB 2016a)

Created in MATLAB 2016a:

                    FilterFunction: @filterIIR
                  FiltfiltFunction: @filtfiltIIR
GetCoefficientsForAnalysisFunction: @getCoeffsForAnalysisIIR


The fields are now present in the filter object, and the code works now.