Plugin can crash during loading if canvas is not created before loading custom parameters

[bparks13]

In some specific instances, the GUI can crash during loading if the editor parameter WINDOW Active="0" is set, but there are X/Y/width/height parameters set. This can be temporarily fixed by setting the property to be WINDOW Active="1", but there is also (at least) one offending line that can be fixed in the code so that this does not occur no matter what the property is set to.

See the line below for where the canvas should be checked to see if it exists before using it so that the GUI does not crash.

onix-source/Source/OnixSourceEditor.cpp

Line 425 in 4dcf5dd

canvas->removeTabs(currentPort);

[bparks13]

During testing, this has shown that there are more issues related to saving/loading devices if the canvas is fully closed. Further testing is required to ensure that all use cases are fully functional