missing text in top windows - cannot save

64bit version This happens intermittently.Editing away and decide to save the file or use any function on the top line of the screen - the text in the panels are missing ( file, edit view etc appear and the windows associated with them appear but there is no text displayed) Cannot Save, Insert, Format… because there is nothing to click! All other panels are blank as well when this happens. Running on Mint 18. Installed software is Version:
Build ID: 1:5.1.6~rc2-0ubuntu1~xenial1
CPU Threads: 2; OS Version: Linux 4.4; UI Render: default;
Locale: en-US (en_US.UTF-8); Calc: group

Not a solution but until the problem is resolved you can save using Ctrl-S or Save As with Shift-Ctrl-S