Table: fix a script error when show-header is true. (#1248)

packages/table/src/table.vue

@@ -194,11 +194,12 @@
         const { bodyWrapper, headerWrapper } = this.$refs;
         const refs = this.$refs;
         bodyWrapper.addEventListener('scroll', function() {
-          headerWrapper.scrollLeft = this.scrollLeft;
+          if (headerWrapper) headerWrapper.scrollLeft = this.scrollLeft;
           if (refs.fixedBodyWrapper) refs.fixedBodyWrapper.scrollTop = this.scrollTop;
           if (refs.rightFixedBodyWrapper) refs.rightFixedBodyWrapper.scrollTop = this.scrollTop;
         });
 
+        if (headerWrapper) {
           mousewheel(headerWrapper, throttle(16, function(event) {
             const deltaX = event.deltaX;
 
@@ -208,6 +209,7 @@
               bodyWrapper.scrollLeft = bodyWrapper.scrollLeft - 10;
             }
           }));
+        }
 
         if (this.fit) {
           this.windowResizeListener = throttle(50, () => {