Fix panic on nil interface conversion (#34)

Signed-off-by: Dmytro Vasylyshyn <vasilishin.d.o@gmail.com>

invoker.go

@@ -45,8 +45,8 @@ func (i *invoker) Invoke(fn any) (InvokeResult, error) {
 		if index == len(fnOutArgs)-1 {
 			// And type of the value is the error.
 			if fnOut.Type().Implements(errorType) {
-				// Use the value as an error.
-				result.err = fnOut.Interface().(error)
+				// Use the value as an error. Perform safe type conversion to avoid panics.
+				result.err, _ = fnOut.Interface().(error)
 			}
 		}
 

invoker_test.go

@@ -66,6 +66,16 @@ func TestInvokerService(t *testing.T) {
 			},
 			wantErr: false,
 		},
+		{
+			name:   "ReturnNilError",
+			haveFn: func(var1 string, var2 int) error { return nil },
+			wantFn: func(t *testing.T, value InvokeResult) {
+				equal(t, len(value.Values()), 1)
+				equal(t, value.Values()[0], nil)
+				equal(t, value.Error(), nil)
+			},
+			wantErr: false,
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {