Check whether the ShaderManager is valid in Explosion effect

The effect did not check whether the ShaderManager is valid causing
a null pointer access when trying to use the shader.

Additionally this change moves the Shader init into the close window
slot as close window referrenced windows without checking whether the
effect would work. If the effect would not work each closed window
would be referrenced without any chance to being unreffed again as this
code is in a block checking whether the effect is valid.


FIXED-IN: 4.8.0