Fix crashes when user hasn't loaded and when postDraft was undefined

app/components/autocomplete/at_mention/index.js

@@ -22,7 +22,7 @@ function mapStateToProps(state, ownProps) {
     const {cursorPosition, isSearch, rootId} = ownProps;
     const currentChannelId = getCurrentChannelId(state);
 
-    let postDraft;
+    let postDraft = '';
     if (isSearch) {
         postDraft = state.views.search;
     } else if (ownProps.rootId) {

app/components/autocomplete/channel_mention/index.js

@@ -23,7 +23,7 @@ function mapStateToProps(state, ownProps) {
     const {cursorPosition, isSearch, rootId} = ownProps;
     const currentChannelId = getCurrentChannelId(state);
 
-    let postDraft;
+    let postDraft = '';
     if (isSearch) {
         postDraft = state.views.search;
     } else if (rootId) {

app/components/post_profile_picture/index.js

@@ -4,7 +4,6 @@
 import {connect} from 'react-redux';
 
 import {getPost} from 'mattermost-redux/selectors/entities/posts';
-import {getUser} from 'mattermost-redux/selectors/entities/users';
 import {isSystemMessage} from 'mattermost-redux/utils/post_utils';
 
 import {getTheme} from 'app/selectors/preferences';
@@ -14,7 +13,6 @@ import PostProfilePicture from './post_profile_picture';
 function mapStateToProps(state, ownProps) {
     const {config} = state.entities.general;
     const post = getPost(state, ownProps.postId);
-    const user = getUser(state, post.user_id);
 
     return {
         ...ownProps,
@@ -22,7 +20,7 @@ function mapStateToProps(state, ownProps) {
         fromWebHook: post.props && post.props.from_webhook === 'true',
         isSystemMessage: isSystemMessage(post),
         overrideIconUrl: post.props && post.props.override_icon_url,
-        user,
+        userId: post.user_id,
         theme: getTheme(state)
     };
 }

app/components/post_profile_picture/post_profile_picture.js

@@ -20,7 +20,7 @@ function PostProfilePicture(props) {
         onViewUserProfile,
         overrideIconUrl,
         theme,
-        user
+        userId
     } = props;
     if (isSystemMessage) {
         return (
@@ -51,7 +51,7 @@ function PostProfilePicture(props) {
 
         return (
             <ProfilePicture
-                userId={user.id}
+                userId={userId}
                 size={PROFILE_PICTURE_SIZE}
             />
         );
@@ -60,7 +60,7 @@ function PostProfilePicture(props) {
     return (
         <TouchableOpacity onPress={onViewUserProfile}>
             <ProfilePicture
-                userId={user.id}
+                userId={userId}
                 size={PROFILE_PICTURE_SIZE}
             />
         </TouchableOpacity>
@@ -74,7 +74,7 @@ PostProfilePicture.propTypes = {
     overrideIconUrl: PropTypes.string,
     onViewUserProfile: PropTypes.func,
     theme: PropTypes.object,
-    user: PropTypes.object
+    userId: PropTypes.string
 };
 
 PostProfilePicture.defaultProps = {