Server error 500 - Value cannot be null when editing pages in CMS

I'm getting this error when trying to edit pages in the CMS. I debugged to the fullest of my knowledge using the Network monitor tab in Chrome's DevTools, googling around and what not.

I've seen quite a few other topics that indicate fixes being released in newer versions of TinyMCS, CMS.UI packages. I'm trying to understand what caused this error in my Preproduction environment and NOT in my Integration environment as they have the same versions of the EPiServer packages in tehri respective environment.

Any idea how I can better pinpoint the root cause before blindly trying to update packages and hoping for a fix?

Thanks,

Here's the error:

"Unable to load /episerver/shell/Stores/metadata/EPiServer.Core.ContentData?modelAccessor=%7B%22contentLink%22%3A%227168_55698%22%7D&dojo.preventCache=1551364610868 status: 500"

And the full stack below when opening the page:

Server Error in '/' Application.
Value cannot be null.
Parameter name: key
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.ArgumentNullException: Value cannot be null.
Parameter name: key

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: 


[ArgumentNullException: Value cannot be null.
Parameter name: key]
   System.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument) +52
   System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add) +795
   EPiServer.Shell.ObjectEditing.EditorDescriptors.EditorDescriptor.SetEditorConfiguration(ExtendedMetadata metadata) +139
   EPiServer.Shell.ObjectEditing.EditorDescriptors.EditorDescriptor.ModifyMetadata(ExtendedMetadata metadata, IEnumerable`1 attributes) +152
   EPiServer.Shell.ObjectEditing.ExtensibleMetadataProvider.ApplyExtendersToMetadata(ExtendedMetadata metadata, IEnumerable`1 attributes, IEnumerable`1 metadataHandlers) +141
   EPiServer.Shell.ObjectEditing.ExtensibleMetadataProvider.GetMetadataForProperties(ExtendedMetadata parent, Object container, Type containerType, IMetadataProvider customProvider) +647
   EPiServer.Shell.ObjectEditing.ExtendedMetadata.get_Properties() +115
   EPiServer.Cms.Shell.UI.ObjectEditing.AllowedTypesMetadataExtender.ModifyMetadata(ExtendedMetadata metadata, IEnumerable`1 attributes) +43
   EPiServer.Shell.ObjectEditing.ExtensibleMetadataProvider.ApplyExtendersToMetadata(ExtendedMetadata metadata, IEnumerable`1 attributes, IEnumerable`1 metadataHandlers) +141
   EPiServer.Shell.ObjectEditing.ExtensibleMetadataProvider.CreateMetadata(IEnumerable`1 attributes, Type containerType, Func`1 modelAccessor, Type modelType, String propertyName) +566
   System.Web.Mvc.AssociatedMetadataProvider.GetMetadataForType(Func`1 modelAccessor, Type modelType) +124
   EPiServer.Shell.UI.Rest.MetadataStore.Get(String id, String modelAccessor) +378
   lambda_method(Closure , ControllerBase , Object[] ) +191
   System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +209
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +35
   System.Web.Mvc.Async.AsyncControllerActionInvoker.<BeginInvokeSynchronousActionMethod>b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +40
   System.Web.Mvc.Async.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult) +77
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +43
   System.Web.Mvc.Async.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d() +72
   System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +387
   System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +387
   System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +387
   System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +387
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +43
   System.Web.Mvc.Async.<>c__DisplayClass2b.<BeginInvokeAction>b__1c() +30
   System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult) +188
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +39
   EPiServer.Shell.Services.Rest.RestControllerBase.EndExecute(IAsyncResult asyncResult) +60
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +648
   System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step) +213
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +131