ROOT crashes with no helpful error messages if a TApplication is not present and tree->StartViewer() is called

[eguiraud]

Describe the bug

The following reproducer crashes with the stacktrace below (with a debug build of ROOT -- just a segfault for non-debug builds):

#include <TTree.h>
#include <TFile.h>
#include <TApplication.h>

int main()
{
   //TApplication app("app", nullptr, nullptr);
   TFile *outputFile = new TFile("output.root", "RECREATE");

   TTree *tree = new TTree("tree", "treeliberated tree");
   outputFile->Write();

   tree->StartViewer();

   //app.Run();
   outputFile->Close();
}

#8  0x00007fd6cf22b8c2 in TGCompositeFrame::TGCompositeFrame (this=0x56036dc34a10, p=0x0, w=10, h=10, options=3, back=0) at ../../../root_dbg/gui/gui/src/TGFrame.cxx:853
#9  0x00007fd6cf22d4cc in TGMainFrame::TGMainFrame (this=0x56036dc34a10, p=0x0, w=10, h=10, options=2) at ../../../root_dbg/gui/gui/src/TGFrame.cxx:1430
#10 0x00007fd6d64b9a6c in TTreeViewer::TTreeViewer (this=0x56036dc34a10, tree=0x56036d05df80) at ../../../root_dbg/tree/treeviewer/src/TTreeViewer.cxx:349
#11 0x00007fd6d809e059 in ?? ()
#12 0x000056036c1be340 in ?? ()
#13 0x000056036c1be340 in ?? ()
#14 0x000056036dc34a10 in ?? ()
#15 0x000056036dc34a10 in ?? ()
#16 0x00007fff775fd220 in ?? ()
#17 0x00007fff775fd3d8 in ?? ()
#18 0x00007fff775fdc20 in ?? ()
#19 0x00007fff775fd3f8 in ?? ()
#20 0x00000001775fd3e0 in ?? ()
#21 0x0000000000000000 in ?? ()

Expected behavior

The crash is due to a nullptr dereference in TGCompositeFrame::TGCompositeFrame (fParent is null), which in turn is caused by a failed check in TGWindow::TGWindow at TGWindow.cxx:54:

if (!p && gClient) {
   p = gClient->GetRoot();
}

In the condition above, if the user created no TApplication, gClient is false-y.

Rather than crash and burning, we could detect the situation and produce a helpful error message.

Additional context

At least one user could have used a better error message 😄