Fix #908: editor throwing an exception when switching from `'preview'` to `'code'` mode

HISTORY.md

@@ -3,6 +3,12 @@
 https://github.com/josdejong/jsoneditor
 
 
+## not yet published, version 8.6.1
+
+- Fix #908: editor throwing an exception when switching from `'preview'`
+  to `'code'` mode.
+
+
 ## 2020-02-16, version 8.6.0
 
 - Fixed #906: Implemented turning Python objects containing `True`, `False` 

docs/api.md

@@ -230,7 +230,7 @@ Constructs a new JSONEditor.
 
 - `{boolean} history`
 
-  Enables history, adds a button Undo and Redo to the menu of the JSONEditor. `true` by default. Only applicable when `mode` is 'tree' or 'form'.
+  Enables history, adds a button Undo and Redo to the menu of the JSONEditor. `true` by default. Only applicable when `mode` is `'tree'`, `'form'`, or `'preview'`.
 
 - `{String} mode`
 

src/js/previewmode.js

@@ -424,8 +424,10 @@ previewmode.destroy = function () {
 
   this._debouncedValidate = null
 
-  this.history.clear()
+  if (this.history) {
-  this.history = null
+    this.history.clear()
+    this.history = null
+  }
 
   // Removing the FocusTracker set to track the editor's focus event
   this.frameFocusTracker.destroy()