Fix race condition in litedb-org#2536

This fixes a race condition in BsonMapper, caused by a fix to a
different issue in litedb-org#2493.

It seems like the current approach of checking the dictionary twice is
deliberate. That said, I don't believe reading from a dictionary
that may be in the process of being updated is actually safe to begin
with.

LiteDB/Client/Mapper/BsonMapper.cs

@@ -235,16 +235,14 @@ internal EntityMapper GetEntityMapper(Type type)
         {
             //TODO: needs check if Type if BsonDocument? Returns empty EntityMapper?
 
-            if (!_entities.TryGetValue(type, out EntityMapper mapper))
+            lock (_entities)
             {
-                lock (_entities)
+                if (!_entities.TryGetValue(type, out EntityMapper mapper))
                 {
-                    if (!_entities.TryGetValue(type, out mapper))
-                        return this.BuildAddEntityMapper(type);
+                    return this.BuildAddEntityMapper(type);
                 }
+                return mapper;
             }
-
-            return mapper;
         }
 
         /// <summary>
@@ -396,7 +394,7 @@ protected virtual CreateObject GetTypeCtor(EntityMapper mapper)
                 for (i = 0; i < pars.Length; i++)
                 {
                     ParameterInfo par = pars[i];
-                    MemberMapper  mi  = null;
+                    MemberMapper mi = null;
                     foreach (MemberMapper member in mapper.Members)
                     {
                         if (member.MemberName.ToLower() == par.Name.ToLower() && member.DataType == par.ParameterType)
@@ -405,10 +403,10 @@ protected virtual CreateObject GetTypeCtor(EntityMapper mapper)
                             break;
                         }
                     }
-                    if (mi == null) {break;}
+                    if (mi == null) { break; }
                     paramMap[i] = new KeyValuePair<string, Type>(mi.FieldName, mi.DataType);
                 }
-                if (i < pars.Length) { continue;}
+                if (i < pars.Length) { continue; }
                 CreateObject toAdd = (BsonDocument value) =>
                 Activator.CreateInstance(type, paramMap.Select(x =>
                 this.Deserialize(x.Value, value[x.Key])).ToArray());