msg not crash when options(error=expr) is set

com.oracle.truffle.r.runtime/src/com/oracle/truffle/r/runtime/RError.java

@@ -156,6 +156,10 @@ public final class RError extends RuntimeException {
                 if (errorExpr instanceof RLanguage || errorExpr instanceof RExpression) {
                     FrameInstance frameInstance = Truffle.getRuntime().getCurrentFrame();
                     VirtualFrame frame = (VirtualFrame) frameInstance.getFrame(FrameAccess.NONE, true);
+                    if (frame == null) {
+                        RContext.getInstance().getConsoleHandler().print("Sorry, options(error=expr) is currently not implemented");
+                        throw new RError(null, "");
+                    }
                     if (errorExpr instanceof RLanguage) {
                         RContext.getEngine().eval((RLanguage) errorExpr, frame);
                     } else if (errorExpr instanceof RExpression) {