Bugzilla

A couple of minor errors when interacting with folder mode headers.

```
JavaScript error: chrome://messenger/content/folderPane.js, line 1327:
TypeError: can't access property "isServer", folder is null
```

```
JavaScript error: chrome://global/content/elements/tree.js, line 1254: 
NS_ERROR_XPC_JAVASCRIPT_ERROR_WITH_DETAILS: [JavaScript Error: 
"can't access property "isServer", folder is null" {file: "chrome://messenger/content/folderPane.js" line: 1327}]'
[JavaScript Error: "can't access property "isServer", folder is null" 
{file: "chrome://messenger/content/folderPane.js" line: 1327}]' when calling method: [nsITreeView::toggleOpenState]
```

A couple of minor errors when interacting with folder mode headers.

```
JavaScript error: chrome://messenger/content/folderPane.js, line 1327:
TypeError: can't access property "isServer", folder is null
```

```
JavaScript error: chrome://messenger/content/folderPane.js, line 2824:
TypeError: can't access property "invalidateRow", this._tree is null
```