Fix addChildren throws error when ext-grid is hidden

Close #978

CHANGELOG.md

@@ -1,14 +1,14 @@
-# BRANCH
-  * [DEPRECATED] jQuery UI widget methods: use `tree.METHOD()` instead.
-  * [Fixed] `$.ui.fancytree.getTree()` for Element arg.
-
 # 2.34.0-0 / Unreleased
   * TODO: tree.hasFocus() more reliable.
+  * [DEPRECATED] jQuery UI widget methods:
+    Use `tree.METHOD()` instead of `$().fancytree("METHOD")`.
   * [Added] `tree.debugTime()`, `tree.debugTimeEnd()` for debugging.
-  * [Added] `tree.destroy()` as shortcut for `tree.widget.destroy()`.
+  * [Added] `tree.destroy()` as alternative for `tree.widget.destroy()`.
+  * [Fixed] `$.ui.fancytree.getTree()` for Element arg.
   * [Fixed] #973 when use ext-grid in one tree, other tree not use ext-grid has error on click.
   * [Fixed] #974 ext-grid: too much output in production mode.
   * [Fixed] #975 ext-grid: fix `tree.visitRows()` for empty tree.
+  * [Fixed] #978 addChildren throws error when ext-grid is hidden.
 
 # 2.33.0 / 2019-10-29
   * [Added] event `preInit` (fired before nodes are loaded).

src/jquery.fancytree.grid.js

@@ -281,15 +281,16 @@
 		// Calculate how many rows fit into current container height
 		var $table = this.$container,
 			wrapper = this.scrollWrapper,
-			trHeight = $table
-				.find(">tbody>tr")
-				.first()
-				.height(),
+			trHeight =
+				$table
+					.find(">tbody>tr")
+					.first()
+					.height() || 0,
 			tableHeight = $table.height(),
 			headHeight = tableHeight - this.viewport.count * trHeight,
 			wrapperHeight = wrapper.offsetHeight,
 			free = wrapperHeight - headHeight,
-			newCount = Math.floor(free / trHeight);
+			newCount = Math.floor(free / trHeight) || 0;
 
 		// console.info(
 		// 	"set container height",
@@ -797,7 +798,7 @@
 						// node.warn("nodeRender(): ignoring hidden");
 						return;
 					}
-					node.warn("nodeRender(): creating new TR!");
+					node.debug("nodeRender(): creating new TR.");
 					node.tr = tree.tbody.rows[node._rowIdx - start];
 				}
 				// _assert(