Bug#1145038: libdex-1-1: manuals fails to start: cannot create new instance of invalid (non-instantiatable) type '<invalid>'

Package: libdex-1-1
Version: 1.2~beta-1
Severity: grave
Justification: renders package unusable
Control: affects -1 + manuals

After upgrading libdex-1-1 to version 1.2~beta-1, manuals can no longer 
launch:

(process:1470144): GLib-GObject-ERROR **: 17:18:22.686: cannot create new instance of invalid (non-instantiatable) type '<invalid>'

Thread 1 "manuals" received signal SIGTRAP, Trace/breakpoint trap.
0x00007ffff6f058aa in ?? () from /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
(gdb) bt
#0  0x00007ffff6f058aa in ??? () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#1  0x00007ffff6f0769e in g_log_default_handler () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#2  0x00007ffff6f07a07 in g_logv () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#3  0x00007ffff6f07d83 in g_log () at /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0
#4  0x00007ffff7d00479 in g_type_create_instance () at /usr/lib/x86_64-linux-gnu/libgobject-2.0.so.0
#5  0x00007ffff68654b5 in foundry_init () at ../libfoundry/foundry-init.c:173
#6  0x000055555555f9e4 in main ()

foundry-init.c:173 is the call to dex_promise_new() here:

DexFuture *
foundry_init (void)
{
  FOUNDRY_ENTRY;

  if (g_once_init_enter (&init_promise))
    {
      g_once_init_leave (&init_promise, dex_promise_new ());
      _foundry_init ();
    }

  FOUNDRY_RETURN (dex_ref (DEX_FUTURE (init_promise)));
}

Possibly caused by this:

+ * Dropped the gconstructor for `dex_init()` which means applications must
+   call `dex_init()` at startup themselves from `main()`.

which is an incompatible change.

If it's that, then we'll need to update packages that use libdex to have 
that initialization call, and then give libdex a versioned Breaks on the 
older versions.

Reporting this as RC to prevent the new libdex from migrating before 
we're ready for it.

    smcv