Last 30 messages being muted causes crash when starting up ZT

Steps to repro:

• Mark Stream #Denmark to be muted (Web-App) in User 1 (Eg: Iago).
• In User 2 (Eg: Hamlet), send 30 odd (around 40 to be on safe side) messages to stream #Denmark.
• Open user Iago from ZT causing it to crash.

Complete Traceback:

ERROR:root:Can't set focus, ListBox is empty
Traceback (most recent call last):
  File "/home/sumanthvrao/Git_Hub/zulip-terminal/zulipterminal/cli/run.py", line 238, in main
    autohide_setting).main()
  File "/home/sumanthvrao/Git_Hub/zulip-terminal/zulipterminal/core.py", line 38, in __init__
    self.view = View(self)
  File "/home/sumanthvrao/Git_Hub/zulip-terminal/zulipterminal/ui.py", line 39, in __init__
    super(View, self).__init__(self.main_window())
  File "/home/sumanthvrao/Git_Hub/zulip-terminal/zulipterminal/ui.py", line 84, in main_window
    self.center_panel = self.message_view()
  File "/home/sumanthvrao/Git_Hub/zulip-terminal/zulipterminal/ui.py", line 46, in message_view
    self.search_box)
  File "/home/sumanthvrao/Git_Hub/zulip-terminal/zulipterminal/ui_tools/views.py", line 284, in __init__
    msg_list = MessageView(model)
  File "/home/sumanthvrao/Git_Hub/zulip-terminal/zulipterminal/ui_tools/views.py", line 34, in __init__
    self.set_focus(self.focus_msg)
  File "/home/sumanthvrao/.local/share/virtualenvs/zulip-terminal-sRIAC_7x/lib/python3.6/site-packages/urwid/listbox.py", line 578, in set_focus
    raise IndexError("Can't set focus, ListBox is empty")
IndexError: Can't set focus, ListBox is empty

NOTE: Discussed in #zulip-terminal>Last 30 muted messages