Fixed bug when clearing file, winscope could not allow up/down arrows.

Test: run yarn dev. First launch a file on winscope, clear, then launch a
second, different file. Now, the up/down arrows will be able to function again.
Change-Id: I4140a1a8f8e1c4d9d2cef054456003aec5661412
Merged-In: I4140a1a8f8e1c4d9d2cef054456003aec5661412
diff --git a/tools/winscope/src/App.vue b/tools/winscope/src/App.vue
index 0654f3b..2c6a148 100644
--- a/tools/winscope/src/App.vue
+++ b/tools/winscope/src/App.vue
@@ -86,6 +86,7 @@
   methods: {
     clear() {
       this.files = [];
+      this.activeDataView = null;
     },
     onTimelineItemSelected(index, timelineIndex) {
       this.files[timelineIndex].selectedIndex = index;