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If you get this message every time you restart Dreamweaver, then you've run into a bug where Extension Manager doesn't update the workspace properly when an extension with a floating panel is uninstalled. To remedy this, go to the upper right of Dreamweaver where there is a drop list of workspace names (mine currently says Designer). Click to open the drop list and select: Reset 'Designer' (or whatever your current workspace name is).
Note: If you've modified your workspace and haven't saved it as a new worksapce then your changes will be reset as well.