Fix crash in the LogsViewer accessing non-existing nodes

dashboard/src/components/LogsViewer.vue

@@ -119,12 +119,12 @@ export default {
     let newLogLines = [];
 
     const tmp = document.getElementsByClassName('pankow-main-layout-body')[0];
-    setInterval(() => {
+    this.refreshInterval = setInterval(() => {
       newLogLines = newLogLines.slice(-maxLines);
 
       for (const line of newLogLines) {
         if (lines < maxLines) ++lines;
-        else this.$refs.linesContainer.removeChild(this.$refs.linesContainer.firstChild);
+        else if (this.$refs.linesContainer.firstChild) this.$refs.linesContainer.removeChild(this.$refs.linesContainer.firstChild);
 
         const logLine = document.createElement('div');
         logLine.className = 'log-line';
@@ -143,6 +143,9 @@ export default {
     }, function (error) {
       newLogLines.push({ time: error.time, html: error.html });
     });
+  },
+  unmounted() {
+    clearInterval(this.refreshInterval);
   }
 };