Unable to view python toolbox tool's parameter properties

[DonMorrison1]

I've been fighting with a problem where I could not package one of my tools as a geoprocessing package. After lots of trial and error I found that it was connected to a totally innocuous try/except in my code.  When the try/except is in the code (see below - for illustration purposes only),  the publishing fails with an error message that provides no clue.  I noticed at some point that if I highlight the problem tool in the Catalog then click Properties -> Parameters I get this message

When I remove the try/except, I can view the properties OK, and more importantly the publish works.  I'm not a python expert so maybe there is a good explanation about how a try statement like that could have such a side effect - I'd love to hear it. 

For my tool I was able to do some recoding and get around the problem.  But if you are having trouble with publishing, perhaps a good test is to make sure the tools parameters are viewable via the tool properties.

import arcpy


class Toolbox(object):
    def __init__(self):
        self.label = "Toolbox"
        self.alias = ""
        self.tools = [TestBoolean]


class TestBoolean(object):
    def __init__(self):
        self.label = "Tool"
        self.description = ""
        self.canRunInBackground = False

    def getParameterInfo(self):
        layer =  arcpy.Parameter(displayName="Feature Layer", 
                name="layer", 
                datatype="GPFeatureLayer",  
                parameterType="Required",  
                direction="Input")                
        checkbox =  arcpy.Parameter(displayName="Checkbox", 
                name="checkbox", 
                datatype="GPBoolean",  
                parameterType="Required",  
                direction="Input")                
        checkbox.value = False
        
        params = [layer, checkbox]
        
        for param in params:
            try:
                filter_list = str(param.filter.list)
            except Exception:
                filter_list = 'Can not access'

        return params

    def isLicensed(self):
        return True

    def updateParameters(self, parameters):
            
        return

    def updateMessages(self, parameters):
        return

    def execute(self, parameters, messages):
        return‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

I'm running this on ArcPro  2.4.3  and Python 3.6.8 and Advanced license.