Add test for indexer initializers

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/GuessAccessors.cs

@@ -97,6 +97,15 @@ namespace ClassLibrary1
 			UnknownStaticGenericClass<string>.OnEvent -= Instance_OnEvent;
 		}
 
+		public void MethodUnknownIndexerInitializer()
+		{
+			//IL_0006: Unknown result type (might be due to invalid IL or missing references)
+			new UnknownClass {
+				["a"] = 1,
+				["b"] = 2
+			};
+		}
+
 		private void Instance_OnEvent(object sender, EventArgs e)
 		{
 			throw new NotImplementedException();

ICSharpCode.Decompiler.Tests/TestCases/ILPretty/GuessAccessors.il

@@ -378,6 +378,28 @@
 		IL_0098: ret
 	} // end of method UnknownClassTest::MethodUnknownStaticGeneric
 
+	.method public hidebysig 
+		instance void MethodUnknownIndexerInitializer () cil managed 
+	{
+		// Method begins at RVA 0x2050
+		// Code size 32 (0x20)
+		.maxstack 8
+
+		IL_0000: nop
+		IL_0001: newobj instance void [UnknownAssembly]UnknownNamespace.UnknownClass::.ctor()
+		IL_0006: dup
+		IL_0007: ldstr "a"
+		IL_000c: ldc.i4.1
+		IL_000d: callvirt instance void [UnknownAssembly]UnknownNamespace.UnknownClass::set_Item(string, int32)
+		IL_0012: nop
+		IL_0013: ldstr "b"
+		IL_0018: ldc.i4.2
+		IL_0019: callvirt instance void [UnknownAssembly]UnknownNamespace.UnknownClass::set_Item(string, int32)
+		IL_001e: nop
+		IL_001f: ret
+	} // end of method C::MethodUnknownIndexerInitializer
+
+
 	.method /* 100663301 */ private hidebysig 
 		instance void Instance_OnEvent (
 			object sender,

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -539,6 +539,8 @@ namespace ICSharpCode.Decompiler.CSharp
 		public ExpressionWithResolveResult BuildDictionaryInitializerExpression(OpCode callOpCode, IMethod method,
 			InitializedObjectResolveResult target, IReadOnlyList<ILInstruction> indices, ILInstruction value = null)
 		{
+			if (method is null)
+				throw new ArgumentNullException(nameof(method));
 			ExpectedTargetDetails expectedTargetDetails = new ExpectedTargetDetails { CallOpCode = callOpCode };
 
 			var callArguments = new List<ILInstruction>();