widgets: Prevent traceback for submessage event.
widgets: Prevent traceback for submessage event.

We can have this scenario:

    - somebody else creates a widget-ready message
    - message arrives in storage
    - (message is not yet in view, so no message.widget)
    - new submessage event arrives

We want to just ignore submessage events in that case.

(There's a more complete fix coming for this scenario, where
we at least update message.submessages for the eventuality
that we do render the message later.)

dhcrzf/zulip,kou/zulip,tommyip/zulip,kou/zulip,showell/zulip,synicalsyntax/zulip,synicalsyntax/zulip,showell/zulip,rht/zulip,tommyip/zulip,zulip/zulip,showell/zulip,timabbott/zulip,rishig/zulip,kou/zulip,shubhamdhama/zulip,hackerkid/zulip,brainwane/zulip,brainwane/zulip,zulip/zulip,shubhamdhama/zulip,punchagan/zulip,dhcrzf/zulip,hackerkid/zulip,punchagan/zulip,dhcrzf/zulip,rht/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,timabbott/zulip,rht/zulip,shubhamdhama/zulip,rishig/zulip,synicalsyntax/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,dhcrzf/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,rishig/zulip,brainwane/zulip,synicalsyntax/zulip,timabbott/zulip,rht/zulip,jackrzhang/zulip,andersk/zulip,synicalsyntax/zulip,synicalsyntax/zulip,timabbott/zulip,punchagan/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,showell/zulip,tommyip/zulip,punchagan/zulip,jackrzhang/zulip,eeshangarg/zulip,brainwane/zulip,rishig/zulip,shubhamdhama/zulip,hackerkid/zulip,zulip/zulip,showell/zulip,tommyip/zulip,kou/zulip,shubhamdhama/zulip,zulip/zulip,brainwane/zulip,timabbott/zulip,rishig/zulip,tommyip/zulip,eeshangarg/zulip,zulip/zulip,eeshangarg/zulip,timabbott/zulip,shubhamdhama/zulip,jackrzhang/zulip,showell/zulip,showell/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,kou/zulip,tommyip/zulip,andersk/zulip,brainwane/zulip,brainwane/zulip,dhcrzf/zulip,jackrzhang/zulip,andersk/zulip,tommyip/zulip,rishig/zulip,eeshangarg/zulip,jackrzhang/zulip,zulip/zulip,jackrzhang/zulip,punchagan/zulip,rishig/zulip,dhcrzf/zulip,timabbott/zulip,jackrzhang/zulip,synicalsyntax/zulip,shubhamdhama/zulip,dhcrzf/zulip,rht/zulip,andersk/zulip