... closing one of them crashes GIMP

GimpSymmetry keeps a strong reference to the drawable passed to
gimp_symmetry_set_origin() until the next call to set_origin(), or
until it's destroyed.  This can unnecessarily extend the lifetime
of the drawable.  In particular, it can extend the lifetime of a
floating selection past the destruction of the image mask, during
image destruction, which results in a NULL mask pointer in
gimp_layer_invalidate_boundary(), called when detaching the
floating selection as part of its destructor.

Add gimp_symmetry_clear_origin(), which clears the origin set using
gimp_symmetry_set_origin(), dropping the reference to the drawable,
and call it in gimp_paint_core_paint() after painting.  This avoids
extending the lifetime of the drawable, and fixes the bug.