C#: Fix bindings generator for Callable argument default value

Previously there weren't any Callable arguments with a default value,
but d4dd859991205e6cecfa9a0553b89db47c983d0b introduced one.

modules/mono/editor/bindings_generator.cpp

@@ -3130,8 +3130,18 @@ bool BindingsGenerator::_arg_default_value_from_variant(const Variant &p_val, Ar
 			r_iarg.def_param_mode = ArgumentInterface::NULLABLE_VAL;
 		} break;
 		case Variant::CALLABLE:
+			ERR_FAIL_COND_V_MSG(r_iarg.type.cname != name_cache.type_Callable, false,
+					"Parameter of type '" + String(r_iarg.type.cname) + "' cannot have a default value of type '" + String(name_cache.type_Callable) + "'.");
+			ERR_FAIL_COND_V_MSG(!p_val.is_zero(), false,
+					"Parameter of type '" + String(r_iarg.type.cname) + "' can only have null/zero as the default value.");
+			r_iarg.default_argument = "default";
+			break;
 		case Variant::SIGNAL:
-			CRASH_NOW_MSG("Parameter of type '" + String(r_iarg.type.cname) + "' cannot have a default value.");
+			ERR_FAIL_COND_V_MSG(r_iarg.type.cname != name_cache.type_Signal, false,
+					"Parameter of type '" + String(r_iarg.type.cname) + "' cannot have a default value of type '" + String(name_cache.type_Signal) + "'.");
+			ERR_FAIL_COND_V_MSG(!p_val.is_zero(), false,
+					"Parameter of type '" + String(r_iarg.type.cname) + "' can only have null/zero as the default value.");
+			r_iarg.default_argument = "default";
 			break;
 		default:
 			CRASH_NOW_MSG("Unexpected Variant type: " + itos(p_val.get_type()));

modules/mono/editor/bindings_generator.h

@@ -536,6 +536,8 @@ class BindingsGenerator {
 		StringName type_Object = StaticCString::create("Object");
 		StringName type_RefCounted = StaticCString::create("RefCounted");
 		StringName type_RID = StaticCString::create("RID");
+		StringName type_Callable = StaticCString::create("Callable");
+		StringName type_Signal = StaticCString::create("Signal");
 		StringName type_String = StaticCString::create("String");
 		StringName type_StringName = StaticCString::create("StringName");
 		StringName type_NodePath = StaticCString::create("NodePath");

modules/mono/mono_gd/gd_mono_marshal.cpp

@@ -1686,7 +1686,9 @@ Callable managed_to_callable(const M_Callable &p_managed_callable) {
 		Object *target = p_managed_callable.target ?
 								   unbox<Object *>(CACHED_FIELD(GodotObject, ptr)->get_value(p_managed_callable.target)) :
 								   nullptr;
-		StringName *method_ptr = unbox<StringName *>(CACHED_FIELD(StringName, ptr)->get_value(p_managed_callable.method_string_name));
+		StringName *method_ptr = p_managed_callable.method_string_name ?
+										   unbox<StringName *>(CACHED_FIELD(StringName, ptr)->get_value(p_managed_callable.method_string_name)) :
+										   nullptr;
 		StringName method = method_ptr ? *method_ptr : StringName();
 		return Callable(target, method);
 	}
@@ -1732,7 +1734,9 @@ Signal managed_to_signal_info(const M_SignalInfo &p_managed_signal) {
 	Object *owner = p_managed_signal.owner ?
 							  unbox<Object *>(CACHED_FIELD(GodotObject, ptr)->get_value(p_managed_signal.owner)) :
 							  nullptr;
-	StringName *name_ptr = unbox<StringName *>(CACHED_FIELD(StringName, ptr)->get_value(p_managed_signal.name_string_name));
+	StringName *name_ptr = p_managed_signal.name_string_name ?
+									 unbox<StringName *>(CACHED_FIELD(StringName, ptr)->get_value(p_managed_signal.name_string_name)) :
+									 nullptr;
 	StringName name = name_ptr ? *name_ptr : StringName();
 	return Signal(owner, name);
 }