We recently upgraded Nintex Forms for SharePoint 2013 and have found suddenly that some forms that have been published and unchanged for ages and broken and won't open in any mode (new, edit or view) because of corrupt or broken people picker fields. 

The uls logs showed an error similar to:

Error=Object reference not set to an instance of an object.   at Nintex.Forms.SharePoint.FormControls.PeoplePickerFormControl.EditModeDataBinding(String value)     at Nintex.Forms.SharePoint.FormControls.PeoplePickerFormControl.OnFormControlDataBinding(Object sender, EventArgs e)     at System.Web.UI.WebControls.CompositeControl.DataBind()     at System.Web.UI.Control.DataBindChildren()     at System.Web.UI.Control.DataBind(Boolean raiseOnDataBinding)     at System.Web.UI.Control.DataBindChildren()

The fix came after a call with the amazing Nintex Support (Kavin) who told us to do the following with the people picker controls in question:

1. Change the control to be unconnected
2. Open the "Advanced" settings and remove the filtering by SharePoint group
3. Reconnect the control
4. Publish