Dear experts

I was trying to setup a fit with ROOT 6.26/00 (currently checking if the segfault goes away in latest ROOT (6.28)

Here a minimal example

```
import ROOT as r
massVariable = r.RooRealVar("m","m",1200)
bComb1 = r.RooRealVar('bcomb1', 'bcomb1', -1E-3, -1E-2, 1E-2)
nComb1 = r.RooRealVar("nComb1","nComb1", 200)
pdf1 = r.RooExtendPdf( "combo1_ext","combo1_ext", r.RooExponential(f"combo1", f"combo1", massVariable , bComb1), nComb1)
pdf1.Print("v")
bComb2 = r.RooRealVar('bcomb2', 'bcomb2', -1E-3, -1E-2, 1E-2)
nComb2 = r.RooRealVar("nComb2","nComb2", 200)
pdf2 = r.RooExtendPdf( "combo2_ext","combo2_ext", r.RooExponential(f"combo2", f"combo2", massVariable , bComb2), nComb2)
pdf2.Print("v")
model = r.RooAddPdf("model","model", r.RooArgList(pdf1,pdf2))
model.Print("v")
```

Basically the Print(â€śvâ€ť) segfault with :

```
--- RooAbsArg ---
Value State: DIRTY
Shape State: DIRTY
Attributes: [ServerDied,ServerDied:combo1(55d19d2c1b00)]
Address: 0x55d19d51d4d0
Clients:
Servers:
(0x55d19d2ee670,V-) RooRealVar::nComb1 "nComb1"
Proxies:
pdf -> *** Break *** segmentation violation
===========================================================
There was a crash.
This is the entire stack trace of all threads:
===========================================================
Thread 2 (Thread 0x7f9e57636700 (LWP 5327)):
#0 0x00007f9e837e0de2 in pthread_cond_timedwait
GLIBC_2.3.2 () from /lib64/libpthread.so.0
```

seems like the Printing is not protected if it has some nullptr not associated tot the pdf?

Thanks

Renato