  In this paper, we report our multiwavelength observations of the C4.2
circular-ribbon flare in active region (AR) 12434 on 2015 October 16. The
short-lived flare was associated with positive magnetic polarities and a
negative polarity inside, as revealed by the photospheric line-of-sight
magnetograms. Such magnetic pattern is strongly indicative of a magnetic null
point and spine-fan configuration in the corona. The flare was triggered by the
eruption of a mini-filament residing in the AR, which produced the inner flare
ribbon (IFR) and the southern part of a closed circular flare ribbon (CFR).
When the eruptive filament reached the null point, it triggered null point
magnetic reconnection with the ambient open field and generated the bright CFR
and a blowout jet. Raster observations of the \textit{Interface Region Imaging
Spectrograph} (\textit{IRIS}) show plasma upflow at speed of 35$-$120 km
s$^{-1}$ in the Fe {\sc xxi} 1354.09 {\AA} line ($\log T\approx7.05$) and
downflow at speed of 10$-$60 km s$^{-1}$ in the Si {\sc iv} 1393.77 {\AA} line
($\log T\approx4.8$) at certain locations of the CFR and IFR during the
impulsive phase of flare, indicating explosive chromospheric evaporation.
Coincidence of the single HXR source at 12$-$25 keV with the IFR and
calculation based on the thick-target model suggest that the explosive
evaporation was most probably driven by nonthermal electrons.
