Fix two caps related tracebacks:

Quick fix: What does not exist cannot support anything. Fixes #4031.
Fix variable referencing. Fixes #4076.

src/common/caps.py

@@ -180,7 +180,7 @@ class CapsCache(object):
 			con.discoverInfo(jid, '%s#%s' % (node, hash))
 	
 	def is_supported(self, contact, feature):
-		if not contact.resource:
+		if not contact or not contact.resource:
 			return False
 
 		# FIXME: We assume everything is supported if we got no caps.

src/dialogs.py

@@ -2315,9 +2315,9 @@ class SingleMessageWindow:
 		if gajim.connections[self.account].connected <= 1:
 			# if offline or connecting
 			ErrorDialog(_('Connection not available'),
-		_('Please make sure you are connected with "%s".') % self.account)
+				_('Please make sure you are connected with "%s".') % self.account)
 			return
-		if type(self.to) == type([]):
+		if isinstance(self.to, list):
 			sender_list = [i[0].jid + '/' + i[0].resource for i in self.to]
 		else:
 			sender_list = [self.to_entry.get_text().decode('utf-8')]
@@ -2325,14 +2325,15 @@ class SingleMessageWindow:
 		for to_whom_jid in sender_list:
 			if self.completion_dict.has_key(to_whom_jid):
 				to_whom_jid = self.completion_dict[to_whom_jid].jid
+
 			subject = self.subject_entry.get_text().decode('utf-8')
 			begin, end = self.message_tv_buffer.get_bounds()
 			message = self.message_tv_buffer.get_text(begin, end).decode('utf-8')
 
-			if to_whom_jid.find('/announce/') != -1:
+			if '/announce/' in to_whom_jid:
 				gajim.connections[self.account].send_motd(to_whom_jid, subject,
 					message)
-				return
+				continue
 
 			if self.session:
 				session = self.session

src/roster_window.py

@@ -5493,13 +5493,12 @@ class RosterWindow:
 		if len(contact_list) > 1: # several resources
 			invite_to_new_room_menuitem.set_submenu(self.build_resources_submenu(
 				contact_list, account, self.on_invite_to_new_room, cap=NS_MUC))
+		elif len(list_) == 1 and gajim.capscache.is_supported(contact, NS_MUC):
+			invite_menuitem.set_sensitive(True)
+			invite_to_new_room_menuitem.connect('activate',
+				self.on_invite_to_new_room, list_)
 		else:
-			if gajim.capscache.is_supported(contact, NS_MUC):
-				invite_menuitem.set_sensitive(True)
-				invite_to_new_room_menuitem.connect('activate',
-					self.on_invite_to_new_room, list_)
-			else:
-				invite_menuitem.set_sensitive(False)
+			invite_menuitem.set_sensitive(False)
 		# transform None in 'jabber'
 		c_t = contacts_transport or 'jabber'
 		muc_jid = {}