Okay tihis was not a common situation but resolving this probably helps on related problems.

This is what happened:

I adde the weather plugin and when the settings window appeared i realized that a starter next to the weather plugin was on the wrong position. So I decided to move it first and to continue weatherplugin settings afterwards. 

After right-clicking on my starter and selecting "Move" from the menu, the panel crashed :-)

HTH, Markus