Flyout.show changes flyout definition.

[alschmiedt]

Describe the bug
When flyout.show() is given a custom category it changes the original toolbox tree found on workspace.options.languageTree.

To Reproduce

1. Create a toolbox with the following definition:

var jsonToolbox = {
  "kind": "flyoutToolbox",
  "contents": [
    {
      "kind": "block",
      "type": "controls_if"
    },
    {
      "kind": "category",
      "custom": "VARIABLE"
    }
  ]
}

2. Run Blockly.getMainWorkspace().options.languageTree in the console.
3. Notice how the new toolbox definition now has a button instead of a custom category:

var jsonToolbox = {
 "kind": "flyoutToolbox",
 "contents": [
   {
     "kind": "block",
     "type": "controls_if"
   },
   {
     "kind": "button",
     "callback": "CREATE_VARIABLE"
   }
 ]
}

Expected behavior
The toolbox definition should still have a custom category.

Additional context
This bug is frustrating when creating a simple flyout with a custom category. In this case a user needs to be able to refresh their flyout when the contents in the custom category have changed. They should be able to do this by calling:
workspace.getFlyout().show(workspace.options.languageTree)
However, this doesn't work because the custom category has been replaced by the button so the flyout will not update.