Activities Functionality implementation

Hi All,

I have a requirement to implement activities (as in vendor master) for my new form.I follwed standard codes which they followed...

and found SysPluginFactory classes which is involved...i created my own methods to implement this ...but when opening the association tab in activities form and selecting parent id i was getting "object reference not set error"

I debugged and checked it was causing from SysPluginFactory class which is there in SmmActivityParentLink class

caseIActivities = SysPluginFactory::Instance('Dynamics.AX.Application', classStr(CaseIActivities), metadataCollection);

i was unable to resolve it.has anyone faced this issue or any resolution which would be appreciated.

Thanks.