Unfortunately this is a known issue, you'll find several threads on these forums with users jumping through hoops to work around it, e.g. https://community.safe.com/s/question/0D54Q000080hbnrSAA/datetimeconverter-rejected-port-null

It's regrettable that the issue seems to still be there in 2022.0.

And it would such an incredibly easy fix, so why is it that hard to fix it ?

Actually, it just dawned on me, that it's defined in this file: C:\\Program Files\\FME 2022\\transformers\\fmesuite.fmx

The code seems to be Tcl/Tk, which I never really learned.

Are there any Tcl/Tk programmers out there, that may be able to decipher it and suggest a fix ?

YES!!!!

I did manage to decipher the TclTk code myself, and offer a correction.

This is the original code piece:

if {         if {{$(PASSTHROUGH)} eq {NO} || tstring length $oldAttrVal] > 0} {
            FME_SetAttribute fme_rejection_code "INVALID_INPUT";
            break;
         } elseif {!$oldAttrExists} {
            FME_UnsetAttributes $attr;
         } elseif {!$oldAttrNull} {
            FME_SetAttribute $attr {};
         };
      }

This is my fix:

if {estring length $newAttrVal] == 0} {
         if {{$(PASSTHROUGH)} eq {NO} || bstring length $oldAttrVal] > 0} {
            FME_SetAttribute $attr $oldAttrVal;
            FME_SetAttribute fme_rejection_code "INVALID_INPUT";
            break;
         } elseif {!$oldAttrExists} {
            FME_UnsetAttributes $attr;
         } elseif {!$oldAttrNull} {
;#            FME_SetAttribute $attr {};
            FME_SetAttribute $attr $oldAttrVal;
         };
      }

And it works 

Now I can use the transformers in a cascading manner.

Yippee!

--------------------- edit:

Actually, the fix need only be added to the "INVALID_INPUT" block:

      if {Âstring length $newAttrVal] == 0} {
         if {{$(PASSTHROUGH)} eq {NO} ||  string length $oldAttrVal] > 0} {
            FME_SetAttribute $attr $oldAttrVal;
            FME_SetAttribute fme_rejection_code "INVALID_INPUT";
            break;
         } elseif {!$oldAttrExists} {
            FME_UnsetAttributes $attr;
         } elseif {!$oldAttrNull} {
            FME_SetAttribute $attr {};
         };
      }

And the updated transformer definition can be kept in a separate FMX file in the same folder as fmesuite.fmx. Just remember to increase its version number.

Thanks for sharing!

FYI, the unresolved known issue is FMEENGINE-56103

Hi Dan.

Did you see my fix ?

It's a very small thing to fix in the fmesuite.fmx file, so couldn't it, and similar fixes, be escalated to be fixed asap?

In the meantime:

Will any FMX file in the same folder be read upon start, so one can add custom updated versions as a seperate file?

Hi Dan.

I went ahead and tried it, and yes, a seperate FMX file will indeed be read upon startup, so such customizations can be kept in its own file. Easier to distribute too.

Yes, I saw your fix and I've escalated the issue. Thanks!