DevTools: Do not attempt to create tooltips for cells with null value (e.g. creation nodes).

third_party/WebKit/Source/devtools/front_end/ui_lazy/DataGrid.js

Index: third_party/WebKit/Source/devtools/front_end/ui_lazy/DataGrid.js
diff --git a/third_party/WebKit/Source/devtools/front_end/ui_lazy/DataGrid.js b/third_party/WebKit/Source/devtools/front_end/ui_lazy/DataGrid.js
index 675221b5aae3a41004f02d9f479dcbcba3ca0d6d..a0d168801c08740abcff2514a2c5d4fc83580ff2 100644
--- a/third_party/WebKit/Source/devtools/front_end/ui_lazy/DataGrid.js
+++ b/third_party/WebKit/Source/devtools/front_end/ui_lazy/DataGrid.js
@@ -1481,7 +1481,7 @@ UI.DataGridNode = class extends Common.Object {
     var data = this.data[columnId];
     if (data instanceof Node) {
       cell.appendChild(data);
-    } else {
+    } else if (data !== null) {
       cell.textContent = data;
       if (this.dataGrid._columns[columnId].longText)
         cell.title = data;