The "GenerateJavaStubs" task failed unexpectedly

I am using Xamarin.Forms with local database sqlite-net-pcl, I created a cross-platform project for Android, IOs and two other DLLs to create the classes of the engine and access. I have been running the project without problems but today I got the following error:

Severity  Code    Description Project File    Line    Suppression State
Error     The "GenerateJavaStubs" task failed unexpectedly.
System.IO.FileNotFoundException: Could not load assembly 'System.ObjectModel, Version=4.0.0.0, Culture=neutral,
PublicKeyToken=b03f5f7f11d50a3a'. Perhaps it doesn't exist in the Mono
for Android profile?
File name: 'System.ObjectModel.dll'
   at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.Resolve(AssemblyNameReference
 reference, ReaderParameters parameters)
   at Java.Interop.Tools.Cecil.DirectoryAssemblyResolver.Resolve(AssemblyNameReference
reference)
   at Mono.Cecil.MetadataResolver.Resolve(TypeReference type)
   at Mono.Cecil.TypeReference.Resolve()
   at Java.Interop.Tools.Cecil.TypeDefinitionRocks.<GetTypeAndBaseTypes>d__1.MoveNext()
   at System.Linq.Enumerable.Any[TSource](IEnumerable'1 source, Func'2 predicate)
   at Java.Interop.Tools.Cecil.TypeDefinitionRocks.IsSubclassOf(TypeDefinition
type, String typeName)
   at Java.Interop.Tools.JavaCallableWrappers.JavaTypeScanner.AddJavaTypes(List'1
javaTypes, TypeDefinition type)
   at Java.Interop.Tools.JavaCallableWrappers.JavaTypeScanner.GetJavaTypes(IEnumerable'1
assemblies, IAssemblyResolver resolver)
   at Xamarin.Android.Tasks.GenerateJavaStubs.Run(DirectoryAssemblyResolver
res)
   at Xamarin.Android.Tasks.GenerateJavaStubs.Execute()
   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()texto
destacado
   at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext()   SQLiteProyecto.Android

and as much as I'm looking on the internet I do not give with it. Has this same error come to anyone? Could you solve it? Greetings, Miguel