In Math, there are dropdown buttons in the Format - Fonts (Modify) and Format - Spacing (Category) dialogs. These recently stopped working in 6.1.

To reproduce:
1, Open Math, select Format - Fonts or Format - Spacing menu item
2, Click the Modify or Category buttons. Nothing happens, except for a bit of debug errors on the console.

Expected results: 
The Modify button would drop down, allowing to modify the font settings in pop up dialogs.
The Category button would drop down, allowing to modify the spacing settings in the Spacing dialog.

Actual results: 
This appears on the console on clicking the buttons: 

(soffice:16088): Gtk-CRITICAL **: 10:44:49.138: gtk_bin_get_child: assertion 'GTK_IS_BIN (bin)' failed

(soffice:16088): GLib-GObject-CRITICAL **: 10:44:49.138: g_object_ref: assertion 'G_IS_OBJECT (object)' failed

(soffice:16088): Gtk-CRITICAL **: 10:44:49.138: gtk_container_remove: assertion 'GTK_IS_CONTAINER (container)' failed

(soffice:16088): Gtk-CRITICAL **: 10:44:49.138: gtk_container_add: assertion 'GTK_IS_WIDGET (widget)' failed

(soffice:16088): GLib-GObject-CRITICAL **: 10:44:49.138: g_object_unref: assertion 'G_IS_OBJECT (object)' failed

(soffice:16088): GLib-GObject-CRITICAL **: 10:44:49.141: g_object_ref: assertion 'G_IS_OBJECT (object)' failed

(soffice:16088): Gtk-CRITICAL **: 10:44:49.141: gtk_container_remove: assertion 'GTK_IS_WIDGET (widget)' failed

(soffice:16088): Gtk-CRITICAL **: 10:44:49.141: gtk_container_add: assertion 'GTK_IS_CONTAINER (container)' failed

(soffice:16088): GLib-GObject-CRITICAL **: 10:44:49.141: g_object_unref: assertion 'G_IS_OBJECT (object)' failed

(soffice:16088): Gdk-CRITICAL **: 10:44:49.142: Window 0x33e4880 has not been made visible in GdkSeatGrabPrepareFunc

Version: 6.1.0.0.beta2+
Build ID: bf1e097c16f9dd672ea43234cbe0eb64c82fd37e
CPU threads: 4; OS: Linux 4.15; UI render: default; VCL: gtk3; 
Locale: hu-HU (hu_HU.UTF-8); Calc: group threaded

bibisected using bibisect-linux-64-6.1 to this commit:
https://cgit.freedesktop.org/libreoffice/core/commit/?id=bf1e097c16f9dd672ea43234cbe0eb64c82fd37e

# bad: [e6fe4b0e968694cdfc483bb0bb16b99f27b306cd] source 8158035a44b4577c5a620decd96d2d1ca0c7c3b9
# good: [2c1cccc19f9e70d2ecbc9ba7815abd674ab6d858] source 6eeac3539ea4cac32d126c5e24141f262eb5a4d9
git bisect start 'origin/master' 'oldest'
# good: [a149937f9357fcd3446b44512751568be628b08a] source 10b6529a2abe78aeb650c5e027cdbad4c2a72219
git bisect good a149937f9357fcd3446b44512751568be628b08a
# good: [ade83f7eb41c1082ea8a8febeef3a6ccf357a125] source c42ed19c1dcde6e2c6210e6f8cc90ba8ba654de1
git bisect good ade83f7eb41c1082ea8a8febeef3a6ccf357a125
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git bisect good 8e4328da7e496abc34d1023534aee86f6f3fda3b
# good: [6116ce64bbd6cf12dddd9a63b4005cfbf14214dd] source 9c475e63ec70c9d3daaf8f6c193d478b86c84196
git bisect good 6116ce64bbd6cf12dddd9a63b4005cfbf14214dd
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git bisect good 267492dcce4a81a0f7f3932f8a8983f32317b4fb
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git bisect bad ccd1d04e61b16e15a8dc1870505e289984864109
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git bisect bad 115609c4d3747d3a77b31ee4c62f27030722c4a0
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git bisect bad 5c0d8cdf6b1c4df6d75a2191962ee84ffbd01908
# good: [578a0727f50d4ba77bfec911d15f6dd5a4aadb1d] source d96ba7a7df7d28558adaaf9b28026dd73855c3cb
git bisect good 578a0727f50d4ba77bfec911d15f6dd5a4aadb1d
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git bisect bad 5c6bc1f5af8a8094958544c001ebbe190e54197f
# good: [0a31f6d3e9e56c592fb03a5a63d695f8e812a4c5] source dd645e70108f31aab611634e77c120e5efe52d05
git bisect good 0a31f6d3e9e56c592fb03a5a63d695f8e812a4c5
# first bad commit: [5c6bc1f5af8a8094958544c001ebbe190e54197f] source bf1e097c16f9dd672ea43234cbe0eb64c82fd37e