Description

When adding the first field to a form via the Form Designer, the Collection is read-only error message might appear in the browser:

Collection is read-only.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 
Exception Details: System.NotSupportedException: Collection is read-only.
Source Error: 
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
Stack Trace: 
[NotSupportedException: Collection is read-only.]
   System.Collections.Specialized.NameValueCollection.Add(String name, String value) 
   Sitecore.Forms.Shell.UI.Controls.FormBuilder.AddToSetNewField() 
   Sitecore.Forms.Shell.UI.FormDesigner.AddNewField() 
[TargetInvocationException: Exception has been thrown by the target of an invocation.]
   System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor) 
   System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments) 
   System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) 
   Sitecore.Shell.Framework.Commands.MethodCommandProcessor.Invoke(PipelineArgs args) 
   Sitecore.Nexus.Pipelines.NexusPipelineApi.Resume(PipelineArgs args, Pipeline pipeline) 
   Sitecore.Pipelines.Pipeline.Start(PipelineArgs args, Boolean atomic) 
   Sitecore.Shell.Framework.Commands.MethodCommand.Execute(CommandContext context) 
   Sitecore.Web.UI.Sheer.ClientPage.Dispatch(String command) 
   Sitecore.Web.UI.Sheer.ClientPage.RaiseEvent() 
   Sitecore.Web.UI.Sheer.ClientPage.OnPreRender(EventArgs e) 
   System.Web.UI.Control.PreRenderRecursiveInternal() 
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Note: This affects Web Forms for Marketers (WFFM) 8.2.0 — 8.2.3 versions.

Solution

To resolve the issue, consider one of the following options:

• Upgrade to Sitecore XP 8.2.4 and Web Forms for Marketers 8.2.4 or later, in accordance with Web Forms for Marketers – compatibility tables.
• Download and install the patch compatible with the affected product version: https://github.com/sitecoresupport/Sitecore.Support.112928.139810/releases.