After expanding and collapsing a panel several times the following intermittent error occurs at Line 20, Column 325896 of the Ext.axd javascript resource file...

ext.axd?v=4.8.3:20 Uncaught TypeError: Cannot read property 'style' of null
at ctor.setStyle (ext.axd?v=4.8.3:20)
at ctor.beginLayoutCycle (ext.axd?v=4.8.3:20)
at ctor.resetLayout (ext.axd?v=4.8.3:20)
at ctor.invalidate (ext.axd?v=4.8.3:20)
at ctor.invalidate (ext.axd?v=4.8.3:20)
at ctor.flushInvalidates (ext.axd?v=4.8.3:20)
at (ext.axd?v=4.8.3:20)
at Function.flushLayouts (ext.axd?v=4.8.3:20)
at Function.resumeLayouts (ext.axd?v=4.8.3:20)
at Object.Ext.resumeLayouts (ext.axd?v=4.8.3:20)
setStyle @ ext.axd?v=4.8.3:20
beginLayoutCycle @ ext.axd?v=4.8.3:20
resetLayout @ ext.axd?v=4.8.3:20
invalidate @ ext.axd?v=4.8.3:20
invalidate @ ext.axd?v=4.8.3:20
flushInvalidates @ ext.axd?v=4.8.3:20
run @ ext.axd?v=4.8.3:20
flushLayouts @ ext.axd?v=4.8.3:20
resumeLayouts @ ext.axd?v=4.8.3:20
Ext.resumeLayouts @ ext.axd?v=4.8.3:20
placeholderExpand @ ext.axd?v=4.8.3:20
expand @ ext.axd?v=4.8.3:20
toggleCollapse @ ext.axd?v=4.8.3:20
callback @ ext.axd?v=4.8.3:20
onClick @ ext.axd?v=4.8.3:20
fire @ ext.axd?v=4.8.3:20
fire @ ext.axd?v=4.8.3:20
publish @ ext.axd?v=4.8.3:20
publishDelegatedDomEvent @ ext.axd?v=4.8.3:20
doDelegatedEvent @ ext.axd?v=4.8.3:20
onDelegatedEvent @ ext.axd?v=4.8.3:20