Fix the nil pointer when computing range.

The chartData is init'd to nil and we need to guard it.

Change-Id: I7b0896624028a0fbb7d34a582f66044fb49b6d5b
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/901605
Commit-Queue: Hao Wu <haowoo@google.com>
Commit-Queue: Ashwin Verleker <ashwinpv@google.com>
Reviewed-by: Ashwin Verleker <ashwinpv@google.com>
Auto-Submit: Hao Wu <haowoo@google.com>

perf/modules/plot-summary-sk/plot-summary-sk.ts

diff --git a/perf/modules/plot-summary-sk/plot-summary-sk.ts b/perf/modules/plot-summary-sk/plot-summary-sk.ts
index 43a9e36..5b3c361 100644
--- a/perf/modules/plot-summary-sk/plot-summary-sk.ts
+++ b/perf/modules/plot-summary-sk/plot-summary-sk.ts
@@ -172,6 +172,10 @@
   }
 
   private recomputeRange() {
+    if (!this.currentChartData) {
+      return;
+    }
+
     if (this.isCommitScale) {
       this.commitsStart = this.currentChartData!.start as number;
       this.commitsEnd = this.currentChartData!.end as number;