GlslEditor: QC crashes when closing a file deleted from outside

Steps to reproduce:

• cp share/qtcreator/glsl/glsl_120.frag from QC sources to a tmp directory
• start QC, open the copied file
• when document is displayed, go to cmd line and remove this file
• switch back to QC
• QC should ask whether to save/close the deleted file - hit 'Close'

Current result:

• QC crashes

Note: this does not happen for QC 3.3.1
Additional note: crash can be triggered by invoking auto completion inside glsl files