improve tabs demo (#14701)

components/tabs/demo/custom-add-trigger.md

@@ -55,8 +55,12 @@ class Demo extends React.Component {
       }
     });
     const panes = this.state.panes.filter(pane => pane.key !== targetKey);
-    if (lastIndex >= 0 && activeKey === targetKey) {
-      activeKey = panes[lastIndex].key;
+    if (panes.length && activeKey === targetKey) {
+      if (lastIndex >= 0) {
+        activeKey = panes[lastIndex].key;
+      } else {
+        activeKey = panes[0].key;
+      }
     }
     this.setState({ panes, activeKey });
   }

components/tabs/demo/editable-card.md

@@ -61,8 +61,12 @@ class Demo extends React.Component {
       }
     });
     const panes = this.state.panes.filter(pane => pane.key !== targetKey);
-    if (lastIndex >= 0 && activeKey === targetKey) {
-      activeKey = panes[lastIndex].key;
+    if (panes.length && activeKey === targetKey) {
+      if (lastIndex >= 0) {
+        activeKey = panes[lastIndex].key;
+      } else {
+        activeKey = panes[0].key;
+      }
     }
     this.setState({ panes, activeKey });
   }