Fix #1311: exception being thrown under certain conditions when switc…

…hing from `code` mode to `preview` mode

HISTORY.md

@@ -3,6 +3,12 @@
 https://github.com/josdejong/jsoneditor
 
 
+## not yet published, version 9.4.2
+
+- Fix #1311: exception being thrown under certain conditions when switching 
+  from `code` mode to `preview` mode.
+
+
 ## 2021-04-25, version 9.4.1
 
 - Improvements in the Korean translation. Thanks @luasenvy.

src/js/textmode.js

@@ -412,7 +412,11 @@ textmode._onChange = function () {
   }
 
   // enable/disable undo/redo buttons
-  setTimeout(() => this._updateHistoryButtons())
+  setTimeout(() => {
+    if (this._updateHistoryButtons) {
+      this._updateHistoryButtons()
+    }
+  })
 
   // validate JSON schema (if configured)
   this._debouncedValidate()
@@ -800,7 +804,11 @@ textmode._setText = function (jsonText, clearHistory) {
       })
     }
 
-    setTimeout(() => this._updateHistoryButtons())
+    setTimeout(() => {
+      if (this._updateHistoryButtons) {
+        this._updateHistoryButtons()
+      }
+    })
   }
 
   // validate JSON schema