Server error when switching mode to ReadOnlyMode

Description

SourceTree.lnk Note: This bug does not have exact steps to reproduce. Its happens sometimes.

1. Configure read-only mode for your site by adding this code under appsettings
<add key="episerver:DatabaseMode" value="ReadOnly" />.
2. Go to Edit View > create or edit any form.
3. Do anything on the form => The site notifies you of read-only mode.
4. Edit the web.config file and remove the added configuration.
5. Refresh the browser.

Issue: (this issue only happens occasionally)

Server Error in '/' Application.
 
An item with the same key has already been added.
Exception Details: System.ArgumentException: An item with the same key has already been added.
Stack Trace: 
 
[ArgumentException: An item with the same key has already been added.]
   System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) +56
   System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) +14287305
   EPiServer.Forms.Helpers.Internal.FormsExtensions.GetDerivedTypes(Type baseType, Boolean isCached) +978
   EPiServer.Forms.EditView.FormsModuleViewModel.get_AllowedPreviewableTextBoxTypes() +102
   GetAllowedPreviewableTextBoxTypes(Object ) +125
   Newtonsoft.Json.Serialization.DynamicValueProvider.GetValue(Object target) +114
 
[JsonSerializationException: Error getting value from 'AllowedPreviewableTextBoxTypes' on 'EPiServer.Forms.EditView.FormsModuleViewModel'.]
   Newtonsoft.Json.Serialization.DynamicValueProvider.GetValue(Object target) +274
   Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.CalculatePropertyValues(JsonWriter writer, Object value, JsonContainerContract contract, JsonProperty member, JsonProperty property, JsonContract& memberContract, Object& memberValue) +205
   Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty) +1120
   Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty) +830
   Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value, Type objectType) +425
   Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value, Type objectType) +1234
   EPiServer.Framework.Serialization.Json.Internal.JsonObjectSerializer.Serialize(TextWriter textWriter, Object value) +71
   EPiServer.Shell.Web.Mvc.Html.ScriptExtensions.SerializeObject(HtmlHelper html, Object value, String contentType) +253
   ASP.episerver_shell_views_shared_bootstrapper_aspx.__Render__control5(HtmlTextWriter __w, Control parameterContainer) in C:\Episerver\Feed\EPiServer\Shell\Views\Shared\Bootstrapper.aspx:58
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +263
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +80
   ASP.episerver_shell_views_shared_sleek_master.__Render__control1(HtmlTextWriter __w, Control parameterContainer) in C:\Episerver\Feed\EPiServer\Shell\Views\Shared\Sleek.Master:30
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +263
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +80
   System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) +160
   System.Web.Mvc.ViewPage.Render(HtmlTextWriter writer) +76
   System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) +80
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +8921