Deselect message history message when topic selection changes

Fixes #93

app/src/actions/Tree.ts

@@ -1,5 +1,6 @@
 import * as q from '../../../backend/src/Model'
 import { ActionTypes } from '../reducers/Tree'
+import { ActionTypes as SidebarActionTypes } from '../reducers/Sidebar'
 import { AnyAction, Dispatch } from 'redux'
 import { AppState } from '../reducers'
 import { batchActions } from 'redux-batched-actions'
@@ -40,6 +41,11 @@ const debouncedSelectTopic = debounce((topic: q.TreeNode<TopicViewModel>, dispat
     type: ActionTypes.TREE_SELECT_TOPIC,
   }
 
+  dispatch({
+    type: SidebarActionTypes.SIDEBAR_SET_COMPARE_MESSAGE,
+    message: undefined,
+  })
+
   if (setTopicDispatch) {
     dispatch(batchActions([selectTreeTopicDispatch, setTopicDispatch]))
   } else {