Crash creating new stream message in streams with no previous conversation.

[sumanthvrao]

The c Hot Key works in creating a new stream message works in streams which has previous conversations. However the terminal crashes when used in streams which do not have any previous conversation. Expecting the Key to create a new message with stream set to current one and topic blank.

Steps to reproduce bug in development environment (newest version) :

1. logged in as Iago@zulip.com
2. Use c in to create new messages in streams with no convo like #sales, #support
3. ZT crashes with the following error message

File "/home/sumanthvrao/.local/share/virtualenvs/zulip-terminal-sRIAC_7x/lib/python3.6/site-packages/urwid/container.py", line 1429, in _set_focus_position
    raise IndexError("No Pile child widget at position %s" % (position,))
IndexError: No Pile child widget at position 0
Zulip Terminal has crashed!

A workaround which avoids the error is to change Stream in message header (to #sales for example) when c is used to create new messages in streams with previous conversation.

[amanagr]

@sumanthvrao thanks for reporting this.