Don't crash if a plugin provides no valid object.
(cherry picked from commit a9da8e33efb991c1c985872d6eb7eac4534d0315)

Conflicts:

	core/proxytoolfactory.cpp
Fix re-injecting of the probe.

The plugin manager singleton was still pointing to the previous instance,
shouldn't be a singleton in the first place I think, but that requires a
bit of refactoring first.
Don't leak unused ProbeCreator instances.

For some unknown reason we end with countless of these on Win/MSVC.
Merge remote-tracking branch 'origin/1.3'

Conflicts:
	common/proxytoolfactory.cpp
	core/probe.cpp