Multiple snackbars pop up when toggling in LayerComparer

[jacobwod]

[...]
We have also experienced some bugs with the layercomparer where the button to end the comparison is not visualised.
If the user switches layers to compare the pop up notice is replicated and if they end the comparison after switching layers to compare multiple background layers can end up as active at the same time.

Originally posted by @WCarlsson in #1092 (comment)