Fix NPE when we don't know the sender of an event

Fixes a bug introduced in
https://github.com/matrix-org/matrix-react-sdk/pull/426.

Particularly when we are showing search results, we may not recognise the
sender of an event; attempting to create a MemberAvatar for it will lead to
null-reference errors.

Also a bit of untangling of the logic of needsSenderProfile. Since
https://github.com/matrix-org/matrix-react-sdk/pull/422,
EventTileType.needsSenderProfile was only being called on MessageEvents, and
therefore only returned true. It's a shame to see all this logic going into
EventTile rather than the individual EventTileTypes, but since it's there,
let's not leave the unused logic lying around in the EventTileType
implementations.

src/components/views/messages/MessageEvent.js

@@ -22,12 +22,6 @@ var sdk = require('../../../index');
 module.exports = React.createClass({
     displayName: 'MessageEvent',
 
-    statics: {
-        needsSenderProfile: function() {
-            return true;
-        }
-    },
-
     propTypes: {
         /* the MatrixEvent to show */
         mxEvent: React.PropTypes.object.isRequired,

src/components/views/messages/TextualEvent.js

@@ -24,12 +24,6 @@ import sdk from '../../../index';
 module.exports = React.createClass({
     displayName: 'TextualEvent',
 
-    statics: {
-        needsSenderProfile: function() {
-            return false;
-        }
-    },
-
     render: function() {
         const EmojiText = sdk.getComponent('elements.EmojiText');
         var text = TextForEvent.textForEvent(this.props.mxEvent);
@@ -39,4 +33,3 @@ module.exports = React.createClass({
         );
     },
 });
-

src/components/views/rooms/EventTile.js

@@ -62,7 +62,7 @@ var MAX_READ_AVATARS = 5;
 // '----------------------------------------------------------'
 
 module.exports = React.createClass({
-    displayName: 'Event',
+    displayName: 'EventTile',
 
     statics: {
         haveTileForEvent: function(e) {
@@ -368,7 +368,7 @@ module.exports = React.createClass({
         // room, or emote messages
         var isInfoMessage = (msgtype === 'm.emote' || eventType !== 'm.room.message');
 
-        var EventTileType = sdk.getComponent(eventTileTypes[this.props.mxEvent.getType()]);
+        var EventTileType = sdk.getComponent(eventTileTypes[eventType]);
         // This shouldn't happen: the caller should check we support this type
         // before trying to instantiate us
         if (!EventTileType) {
@@ -395,31 +395,44 @@ module.exports = React.createClass({
                             <MessageTimestamp ts={this.props.mxEvent.getTs()} />
                         </a>
 
-        var aux = null;
-        if (msgtype === 'm.image') aux = "sent an image";
-        else if (msgtype === 'm.video') aux = "sent a video";
-        else if (msgtype === 'm.file') aux = "uploaded a file";
-
         var readAvatars = this.getReadAvatars();
 
         var avatar, sender;
+        let avatarSize;
+        let needsSenderProfile;
+
         if (isInfoMessage) {
+            // a small avatar, with no sender profile, for emotes and
+            // joins/parts/etc
+            avatarSize = 14;
+            needsSenderProfile = false;
+        } else if (this.props.continuation) {
+            // no avatar or sender profile for continuation messages
+            avatarSize = 0;
+            needsSenderProfile = false;
+        } else {
+            avatarSize = 30;
+            needsSenderProfile = true;
+        }
+
+        if (this.props.mxEvent.sender && avatarSize) {
             avatar = (
-               <div className="mx_EventTile_avatar">
-                   <MemberAvatar member={this.props.mxEvent.sender} width={14} height={14} onClick={ this.onMemberAvatarClick } />
-               </div>
-            );
-        } else if (!this.props.continuation) {
-            if (this.props.mxEvent.sender) {
-                avatar = (
                     <div className="mx_EventTile_avatar">
-                        <MemberAvatar member={this.props.mxEvent.sender} width={30} height={30} onClick={ this.onMemberAvatarClick } />
+                        <MemberAvatar member={this.props.mxEvent.sender}
+                            width={avatarSize} height={avatarSize}
+                            onClick={ this.onMemberAvatarClick }
+                        />
                     </div>
-                );
-            }
-            if (EventTileType.needsSenderProfile()) {
-                sender = <SenderProfile onClick={ this.onSenderProfileClick } mxEvent={this.props.mxEvent} aux={aux} />;
-            }
+            );
+        }
+
+        if (needsSenderProfile) {
+            let aux = null;
+            if (msgtype === 'm.image') aux = "sent an image";
+            else if (msgtype === 'm.video') aux = "sent a video";
+            else if (msgtype === 'm.file') aux = "uploaded a file";
+
+            sender = <SenderProfile onClick={ this.onSenderProfileClick } mxEvent={this.props.mxEvent} aux={aux} />;
         }
 
         var editButton = (