CSHARP-5139: Coverity analysis defect 127428: Explicit null dereferenced.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/MethodTranslators/WindowMethodToAggregationExpressionTranslator.cs

@@ -292,6 +292,7 @@ public static AggregationExpression Translate(TranslationContext context, Method
 
                 if (method.IsOneOf(__unaryMethods))
                 {
+                    ThrowIfSelectorTranslationIsNull(selectorTranslation);
                     var @operator = GetUnaryWindowOperator(method);
                     var ast = AstExpression.UnaryWindowExpression(@operator, selectorTranslation.Ast, window);
                     var serializer = BsonSerializer.LookupSerializer(method.ReturnType); // TODO: use correct serializer
@@ -313,6 +314,7 @@ public static AggregationExpression Translate(TranslationContext context, Method
 
                 if (method.IsOneOf(__derivativeOrIntegralMethods))
                 {
+                    ThrowIfSelectorTranslationIsNull(selectorTranslation);
                     WindowTimeUnit? unit = default;
                     if (HasArgument<Expression>(parameters, "unit", arguments, out var unitExpression))
                     {
@@ -327,6 +329,7 @@ public static AggregationExpression Translate(TranslationContext context, Method
 
                 if (method.IsOneOf(__exponentialMovingAverageMethods))
                 {
+                    ThrowIfSelectorTranslationIsNull(selectorTranslation);
                     var weightingExpression = arguments[2];
                     var weighting = weightingExpression.GetConstantValue<ExponentialMovingAverageWeighting>(expression);
 
@@ -337,6 +340,7 @@ public static AggregationExpression Translate(TranslationContext context, Method
 
                 if (method.IsOneOf(__shiftMethods))
                 {
+                    ThrowIfSelectorTranslationIsNull(selectorTranslation);
                     var byExpression = arguments[2];
                     var by = byExpression.GetConstantValue<int>(expression);
 
@@ -435,6 +439,14 @@ private static bool HasArgument<TExpression>(ParameterInfo[] parameters, string
             return false;
         }
 
+        private static void ThrowIfSelectorTranslationIsNull(AggregationExpression selectTranslation)
+        {
+            if (selectTranslation == null)
+            {
+                throw new Exception("selectorTranslation is unexpectedly null.");
+            }
+        }
+
         private static AggregationExpression TranslateSelector(TranslationContext context, LambdaExpression selectorLambda, IBsonSerializer inputSerializer)
         {
             var selectorParameter = selectorLambda.Parameters[0];